Fix JavaScriptCodeWithScope serialization

According to the spec:
    code_w_s 	::= 	int32 string document 	Code w/ scope
where the integer represents the entire length of the code_w_s object.
Because an int32 takes 4 bytes, we must add +4 to the length of the
string and document, not just +1.

This behavior was confirmed using the go-mgo driver.

Author: alanhdu

src/encoder/mod.rs

@@ -117,7 +117,7 @@ fn encode_bson<W: Write + ?Sized>(writer: &mut W, key: &str, val: &Bson) -> Enco
             try!(write_string(&mut buf, code));
             try!(encode_document(&mut buf, scope));
 
-            try!(write_i32(writer, buf.len() as i32 + 1));
+            try!(write_i32(writer, buf.len() as i32 + 4));
             writer.write_all(&buf).map_err(From::from)
         },
         &Bson::I32(v) => write_i32(writer, v),

tests/modules/encoder_decoder.rs

@@ -136,21 +136,21 @@ fn test_encode_decode_javascript_code() {
     assert_eq!(decoded, doc);
 }
 
-//#[test]
-//fn test_encode_decode_javascript_code_with_scope() {
-//    let src = Bson::JavaScriptCodeWithScope("1".to_owned(), doc!{});
-//    let dst = vec![25, 0, 0, 0, 15, 107, 101, 121, 0, 12, 0, 0, 0, 2, 0, 0, 0, 49, 0, 5, 0, 0, 0, 0, 0];
-//
-//    let doc = doc!{ "key" => src };
-//
-//    let mut buf = Vec::new();
-//    encode_document(&mut buf, &doc).unwrap();
-//
-//    assert_eq!(buf, dst);
-//
-//    let decoded = decode_document(&mut Cursor::new(buf)).unwrap();
-//    assert_eq!(decoded, doc);
-//}
+#[test]
+fn test_encode_decode_javascript_code_with_scope() {
+    let src = Bson::JavaScriptCodeWithScope("1".to_owned(), doc!{});
+    let dst = vec![25, 0, 0, 0, 15, 107, 101, 121, 0, 15, 0, 0, 0, 2, 0, 0, 0, 49, 0, 5, 0, 0, 0, 0, 0];
+
+    let doc = doc!{ "key" => src };
+
+    let mut buf = Vec::new();
+    encode_document(&mut buf, &doc).unwrap();
+
+    assert_eq!(buf, dst);
+
+    //let decoded = decode_document(&mut Cursor::new(buf)).unwrap();
+    //assert_eq!(decoded, doc);
+}
 
 #[test]
 fn test_encode_decode_i32() {