Restore support for legacy unquoted string literals

While we definitely want to transition away from unquoted string
literals, it seems reasonable to do that over time, similarly to how it
was handled in jmespath.py.

The reason for doing so, is to allow for a simple upgrade path, where a
new version of jmespath.rb can be installed, and the software using it
can transition all its configuration to use the new format, and then the
jmespath.rb version could be upgraded to not even support these
expressions.

The tests were copied verbatim from jmespath.py, and are not included in
the compliance test suite, so comes with an explicit license, which the
compliance test suite does not. JSON doesn't allow for comments, so I'm
not sure how to handle the attribution clause of the jmespath.py license

lib/jmespath/lexer.rb

@@ -243,7 +243,7 @@ def tokenize(expression)
           token = inside(chars, '"', T_QUOTED_IDENTIFIER)
           if token.type == T_QUOTED_IDENTIFIER
             token.value = "\"#{token.value}\""
-            token = parse_json(token)
+            token = parse_json(token, true)
           end
           tokens << token
         when STATE_EQ
@@ -295,9 +295,13 @@ def inside(chars, delim, type)
       Token.new(type, buffer.join, position)
     end
 
-    def parse_json(token)
+    def parse_json(token, quoted=false)
       begin
-        token.value = JSON.load(token.value)
+        if quoted
+          token.value = JSON.load(token.value)
+        else
+          token.value = JSON.load(token.value) rescue JSON.load(sprintf('"%s"', token.value.lstrip))
+        end
       rescue JSON::ParserError
         token.type = T_UNKNOWN
       end

spec/compliance_spec.rb

@@ -5,7 +5,7 @@
 SimpleCov.command_name('test:compliance')
 
 describe 'Compliance' do
-  Dir.glob('spec/compliance/*.json').each do |path|
+  Dir.glob('spec/{compliance,legacy}/*.json').each do |path|
 
     test_file = File.basename(path).split('.').first
     next if test_file == 'benchmarks'

spec/legacy/literal.json

@@ -0,0 +1,56 @@
+[
+    {
+        "given": {
+            "foo": [{"name": "a"}, {"name": "b"}],
+            "bar": {"baz": "qux"}
+        },
+        "cases": [
+            {
+                "expression": "`foo`",
+                "result": "foo"
+            },
+            {
+                "comment": "Double quotes must be escaped.",
+                "expression": "`foo\\\"quote`",
+                "result": "foo\"quote"
+            },
+            {
+                "expression": "`✓`",
+                "result": "✓"
+            },
+            {
+                "comment": "Double quote in literal",
+                "expression": "`foo\\\"bar`",
+                "result": "foo\"bar"
+            },
+            {
+                "expression": "`1\\``",
+                "result": "1`"
+            },
+            {
+                "comment": "Multiple literal expressions with escapes",
+                "expression": "`\\\\`.{a:`b`}",
+                "result": {"a": "b"}
+            }
+        ]
+    },
+  {
+    "comment": "Literals",
+    "given": {"type": "object"},
+    "cases": [
+      {
+        "expression": "`foo`",
+        "result": "foo"
+      },
+      {
+        "expression": "`      foo`",
+        "result": "foo"
+      },
+      {
+        "comment": "Literal on RHS of subexpr not allowed",
+        "expression": "foo.`bar`",
+        "error": "syntax"
+      }
+    ]
+  }
+]