NullTranslations.__init__(): Back out of setting the default charset

to iso-8859-1.

GNUTranslations._parse(): Back out the addition of a test for
Project-ID-Version in the metadata.  This was deliberately removed in
response to SF patch #700839.

Also, re-organize the code in _parse() so we parse the metadata header
containing the charset parameter before we try to decode any strings
using charset.