fixed emscripten_async_wget2_data callbacks causing compilation errors i...

Hello,

I had to make this changes to successfully build the html5 framework on Windows with the script: build_html5_no_tutorial_no_example.bat

Otherwise there were a mismatch with the signatures of the callbacks implemented in HTTPProtocol class compared to the ones in the emscripten header file. Not sure whether the wrapper methods are still necessary but at least it did build. :)