Allow application to call nghttp2_submit_data() in on_frame_send_callback

Previously we always call on_frame_send_callback before calling
nghttp2_stream_detach_data() after sending DATA frame.  As a result,
even if DATA frame has END_STREAM, application cannot call
nghttp2_submit_data() in on_frame_send_callback because previous data
is still attached.  This commit makes a change so that
nghttp2_stream_detach_data() is called before on_frame_send_callback
so that application can issue nghttp2_submit_data() in the callback.