#4487: have Charset check with codecs for possible aliases.

#4487: have Charset check with codecs for possible aliases.

Previously, unexpected results occurred when email was passed, for example,
'utf8' as a charset name, since email would accept it but would *not* use
the 'utf-8' codec for it, even though Python itself recognises that as
an alias for utf-8.  Now Charset checks with codecs for aliases as well
as its own internal table.  Issue 8898 has been opened to change this
further in py3k so that all aliasing is routed through the codecs module.