FrameDecoder doesn't handle medium-sized frames properly

[Crokoking]

When FrameDecoder determines the size of a medium-sized payload it uses Integer.toUnsignedLong() instead of Short.toUnsignedLong() potentially causing negative payload sizes and essentially breaking the connection.

[epickrram]

Hi, thanks for the report. We'll look at this when possible. In the mean time, feel free to submit a PR containing a fix.