[GplexBuffers]
   If Seek is not supported on the input stream, and "Guess" is specified,
   fallback to the default system code page.

   For unicode scanners, verify the input stream supports Seek() before
   attempting to recognize a Preamble or Guess the Encoding. Fallback to
   using "raw" bytes or the user's fallback encoding when seek is not
   supported.

   Only use the fallback encoding when we're unable to perform a guess.
Merge branch 'master' into dev

Conflicts:
	GPLEX/Parser.cs
	GPLEX/Scanner.cs