Support incorrectly labeled us-ascii charsets

Some email clients (names omitted to protect the innocent) advertise the
encoding as us-ascii when iso-8859-1 was really implied. This patch allows
the two charsets to be interchangable.
Merge pull request #770 from greezybacon/issue/us-ascii-not-really

Support incorrectly labeled us-ascii charsets

Reviewed-By: Peter Rotich <peter@osticket.com>