api: provides default_value_os and default_value_if[s]_os

One can now define default values that contain invalid UTF-8.

The underlying implementation has also been changed to use OsStrs in order to avoid duplication
of code and provide the new APIs basically for free.

Closes #849

src/app/help.rs

@@ -491,12 +491,12 @@ impl<'a> Help<'a> {
         debugln!("Help::spec_vals: a={}", a);
         let mut spec_vals = vec![];
         if let Some(pv) = a.default_val() {
-            debugln!("Help::spec_vals: Found default value...[{}]", pv);
+            debugln!("Help::spec_vals: Found default value...[{:?}]", pv);
             spec_vals.push(format!(" [default: {}]",
                                    if self.color {
-                                       self.cizer.good(pv)
+                                       self.cizer.good(pv.to_string_lossy())
                                    } else {
-                                       Format::None(pv)
+                                       Format::None(pv.to_string_lossy())
                                    }));
         }
         if let Some(ref aliases) = a.aliases() {

src/app/mod.rs

@@ -1558,8 +1558,8 @@ impl<'n, 'e> AnyArg<'n, 'e> for App<'n, 'e> {
     fn val_delim(&self) -> Option<char> { None }
     fn takes_value(&self) -> bool { true }
     fn help(&self) -> Option<&'e str> { self.p.meta.about }
-    fn default_val(&self) -> Option<&'n str> { None }
-    fn default_vals_ifs(&self) -> Option<vec_map::Values<(&'n str, Option<&'e str>, &'e str)>> {None}
+    fn default_val(&self) -> Option<&'e OsStr> { None }
+    fn default_vals_ifs(&self) -> Option<vec_map::Values<(&'n str, Option<&'e OsStr>, &'e OsStr)>> {None}
     fn longest_filter(&self) -> bool { true }
     fn aliases(&self) -> Option<Vec<&'e str>> {
         if let Some(ref aliases) = self.p.meta.aliases {

src/args/any_arg.rs

@@ -33,8 +33,8 @@ pub trait AnyArg<'n, 'e>: std_fmt::Display {
     fn takes_value(&self) -> bool;
     fn val_names(&self) -> Option<&VecMap<&'e str>>;
     fn help(&self) -> Option<&'e str>;
-    fn default_val(&self) -> Option<&'n str>;
-    fn default_vals_ifs(&self) -> Option<vec_map::Values<(&'n str, Option<&'e str>, &'e str)>>;
+    fn default_val(&self) -> Option<&'e OsStr>;
+    fn default_vals_ifs(&self) -> Option<vec_map::Values<(&'n str, Option<&'e OsStr>, &'e OsStr)>>;
     fn longest_filter(&self) -> bool;
     fn val_terminator(&self) -> Option<&'e str>;
 }