Restore compatibility with rustc 1.31 in RenameRule error

str::escape_debug wasn't stabilized until 1.34, whereas serde_derive
currently supports an oldest version of 1.31.

serde_derive/src/internals/case.rs

@@ -5,7 +5,7 @@
 #[allow(deprecated, unused_imports)]
 use std::ascii::AsciiExt;
 
-use std::fmt::{self, Display};
+use std::fmt::{self, Debug, Display};
 
 use self::RenameRule::*;
 
@@ -120,16 +120,14 @@ pub struct ParseError<'a> {
 
 impl<'a> Display for ParseError<'a> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        f.write_str("unknown rename rule `rename_all = \"")?;
-        self.unknown.escape_debug().fmt(f)?;
-        f.write_str("\"`, expected one of ")?;
+        f.write_str("unknown rename rule `rename_all = ")?;
+        Debug::fmt(self.unknown, f)?;
+        f.write_str("`, expected one of ")?;
         for (i, (name, _rule)) in RENAME_RULES.iter().enumerate() {
             if i > 0 {
                 f.write_str(", ")?;
             }
-            f.write_str("\"")?;
-            name.escape_debug().fmt(f)?;
-            f.write_str("\"")?;
+            Debug::fmt(name, f)?;
         }
         Ok(())
     }