change x-gbk to gbk when decoding headerline.
Brought to you by:
nicolasrougier,
sowadart
some mail client use x-gbk instade of gbk, but gnibuff can't handle this when decoding headerline, so get "Cannot decode this header line". This patch change x-gbk to gbk before g_convert(), and then get correct headerline.
The patch to change x-gbk to gbk in decoding headerline.