Serialize to binary if the serde format is not human readable

This implements the KISS suggested in https://github.com/serde-rs/serde/issues/790.
It is possible that one of the other approaches may be better but this
seemed like the simplest one to reignite som discussion.

Personally I find the original suggestion of adding two traits perhaps slightly
cleaner in theory but I think it ends up more complicated in the end
since the added traits also need to be duplicated to to the `Seed`
traits.

Closes #790

test_suite/tests/test_ser.rs

@@ -23,7 +23,8 @@ use std::str;
 extern crate serde;
 
 extern crate serde_test;
-use self::serde_test::{Token, assert_ser_tokens, assert_ser_tokens_error};
+use self::serde_test::{Token, assert_ser_tokens, assert_ser_tokens_error,
+                       assert_ser_tokens_readable};
 
 extern crate fnv;
 use self::fnv::FnvHasher;
@@ -474,3 +475,26 @@ fn test_enum_skipped() {
         "the enum variant Enum::SkippedMap cannot be serialized",
     );
 }
+
+struct CompactBinary(String);
+
+impl serde::Serialize for CompactBinary {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer
+    {
+        if serializer.is_human_readable() {
+            serializer.serialize_str(&self.0)
+        } else {
+            serializer.serialize_bytes(self.0.as_bytes())
+        }
+    }
+}
+
+#[test]
+fn test_human_readable() {
+    let value = CompactBinary("test".to_string());
+    assert_ser_tokens(&value, &[Token::String("test")]);
+
+    assert_ser_tokens_readable(&value, &[Token::Bytes(b"test")], false);
+}