to make [this line](https://github.com/RobotWebTools/roslibjs/blob/develop/src/core/Ros.js#L186) work.
message is already in stirng format, e.g., [callOnConnection stringify](https://github.com/RobotWebTools/roslibjs/blob/develop/src/core/Ros.js#L182-L193) it already.
[This line](https://github.com/RobotWebTools/roslibjs/blob/develop/src/core/Ros.js#L125) in  parses to json. No need to parse to JSON here.
updated to the same file in develop branch.