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 until in R15. 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