fix(headers): fix broken deserialization of headers

Sorry about that!

src/header/mod.rs

@@ -382,9 +382,7 @@ impl Deserialize for Headers {
             }
         }
 
-        let result = Headers::new();
+        deserializer.visit_map(HeadersVisitor)
-        try!(deserializer.visit_map(HeadersVisitor));
-        Ok(result)
     }
 }