Handle a bug in unicode module in releases R14B04 and previous

The code points 16#FFFE and 16#FFFF (not assigned but valid) are considered
invalid for releases R14B04 and previous. So for these releases, the websockets
testsuite always failed. No, it is fixed.

It was fixed in OTP with the commit 34db767655:
 https://github.com/erlang/otp/commit/34db767655
update known dialyzer warnings [skip ci]