Add Scala 2.13 support

This project now compiles with Scala 2.13 by default, while still
cross-compiling to Scala 2.12 & 2.11.

Issues addressed with this upgrade:

### Underscore (`_`) is no longer a valid identifer for `val`s

Using underscore like this is no longer allowed...

```
implicit val _ = ...
```

...we have to give the `val` a valid name (even if it's just `v`):


```
implicit val v = ...
```

See also:

* scala/bug#10384
* scala/bug#7691
* scala/scala#6218

### `Map.mapValues()` now returns a `MapView` rather than a `Map`

We usually want a `Map`, so we can just add in a `.toMap` to the end of
the expression - it's harmless in Scala 2.12, and allows the code to
compile in Scala 2.13. `Map.mapValues()` itself is deprecated in Scala
2.13, but using the new recommended alternative doesn't work in Scala 2.12.

https://docs.scala-lang.org/overviews/core/collections-migration-213.html#what-are-the-breaking-changes

See also:

* scala/bug#10919
* scala/scala#7014

### `.to` can no longer infer what resulting collection type you want

This is all part of `CanBuildFrom` going away in Scala 2.13:

https://www.scala-lang.org/blog/2018/06/13/scala-213-collections.html#life-without-canbuildfrom

build.sbt

@@ -5,7 +5,7 @@ enablePlugins(GhpagesPlugin, SiteScaladocPlugin)
 name := "twitter4s"
 version := "6.2-SNAPSHOT"
 
-scalaVersion := "2.12.4"
+scalaVersion := "2.13.1"
 
 resolvers ++= Seq(
   Resolver.sonatypeRepo("releases"),
@@ -53,7 +53,7 @@ lazy val standardSettings = Seq(
     ScmInfo(url("https://github.com/DanielaSfregola/twitter4s"),
             "scm:git:[email protected]:DanielaSfregola/twitter4s.git")),
   apiURL := Some(url("http://DanielaSfregola.github.io/twitter4s/latest/api/")),
-  crossScalaVersions := Seq("2.12.4", "2.11.12"),
+  crossScalaVersions := Seq(scalaVersion.value, "2.12.10", "2.11.12"),
   pomExtra := (
     <developers>
     <developer>

src/main/scala/com/danielasfregola/twitter4s/http/clients/Client.scala

@@ -18,7 +18,7 @@ trait Client extends OAuthClient {
   protected def sendAndReceive[T](request: HttpRequest, f: HttpResponse => Future[T])(
       implicit system: ActorSystem,
       materializer: Materializer): Future[T] = {
-    implicit val _ = request
+    implicit val r: HttpRequest = request
     val requestStartTime = System.currentTimeMillis
 
     if (withLogRequest) logRequest

src/main/scala/com/danielasfregola/twitter4s/http/marshalling/BodyEncoder.scala

@@ -42,6 +42,6 @@ trait BodyEncoder {
 
   // TODO - improve performance with Macros?
   private def asMap(cc: Product): Map[String, Any] =
-    cc.getClass.getDeclaredFields.map(_.getName).zip(cc.productIterator.to).toMap
+    cc.getClass.getDeclaredFields.map(_.getName).zip(cc.productIterator.toSeq).toMap
 
 }

src/main/scala/com/danielasfregola/twitter4s/http/oauth/OAuth1Provider.scala

@@ -30,7 +30,7 @@ private[twitter4s] class OAuth1Provider(consumerToken: ConsumerToken, accessToke
     val params = basicOAuth2Params(callback)
     for {
       signature <- oauth1Signature(params)
-    } yield (params + signature).mapValues(_.urlEncoded)
+    } yield (params + signature).mapValues(_.urlEncoded).toMap
   }
 
   def oauth1Signature(oauth2Params: Map[String, String])(implicit request: HttpRequest, materializer: Materializer) = {
@@ -67,7 +67,7 @@ private[twitter4s] class OAuth1Provider(consumerToken: ConsumerToken, accessToke
       val method = request.method.name.urlEncoded
       val baseUrl = request.uri.endpoint.urlEncoded
       val oauthParams = oauth2Params.map {
-        case (k, v) =>
+        case (k: String, v: String) =>
           if (k == "oauth_callback") k -> v.urlEncoded
           else k -> v
       }
@@ -90,7 +90,8 @@ private[twitter4s] class OAuth1Provider(consumerToken: ConsumerToken, accessToke
     }
   }
 
-  def queryParams(implicit request: HttpRequest) = request.uri.query().toMap.mapValues(_.urlEncoded)
+  def queryParams(implicit request: HttpRequest): Map[String, String] =
+    request.uri.query().toMap.mapValues(_.urlEncoded).toMap
 
   private def encodeParams(params: Map[String, String]) =
     params.keySet.toList.sorted

src/main/scala/com/danielasfregola/twitter4s/http/serializers/StreamingMessageFormats.scala

@@ -21,7 +21,7 @@ private[twitter4s] object StreamingMessageFormats extends FormatsComposer {
   private val tweetUnmarshaller = FieldSerializer[Tweet](deserializer = FieldSerializer.renameFrom("full_text", "text"))
 
   private def withCustomUnmarshaller[T <: StreamingMessage: Manifest](json: JValue, formatter: Formats): Option[T] = {
-    implicit val _: Formats = formatter
+    implicit val formats: Formats = formatter
     Extraction.extractOpt[T](json)
   }