add comment for adding Content-Length header

Author: jay-choe

core/src/main/java/feign/Client.java

@@ -233,7 +233,7 @@ else if (field.equals(ACCEPT_ENCODING)) {
       }
 
       if (body == null && request.httpMethod().isWithBody()) {
-        // To use this Header, set sun.net.http.allowRestrictedHeaders true.
+        // To use this Header, set 'sun.net.http.allowRestrictedHeaders' property true.
         connection.addRequestProperty("Content-Length", "0");
       }