Fix: String::from_utf8 changed function signature

Following a rust std change.

src/json/ser.rs

@@ -2,6 +2,7 @@ use std::f32;
 use std::f64;
 use std::num::{Float, FPNaN, FPInfinite};
 use std::io::{IoError, IoResult};
+use std::str::Utf8Error;
 
 use ser::Serialize;
 use ser;
@@ -599,7 +600,7 @@ pub fn to_vec<
 #[inline]
 pub fn to_string<
     T: Serialize<Serializer<Vec<u8>>, IoError>
->(value: &T) -> Result<String, Vec<u8>> {
+>(value: &T) -> Result<String, (Vec<u8>, Utf8Error)> {
     let buf = to_vec(value);
     String::from_utf8(buf)
 }
@@ -628,7 +629,7 @@ pub fn to_pretty_vec<
 /// Encode the specified struct into a json `String` buffer.
 pub fn to_pretty_string<
     T: Serialize<PrettySerializer<Vec<u8>>, IoError>
->(value: &T) -> Result<String, Vec<u8>> {
+>(value: &T) -> Result<String, (Vec<u8>, Utf8Error)> {
     let buf = to_pretty_vec(value);
     String::from_utf8(buf)
 }