Tweet text with non-unibyte characters are properly encoded and inserted...

As with the fix for erc-youtube.el, Emacs 24.3 url-retrieve<f> and friends returns the network data in a unibyte buffer. We force a multibyte interpretation, and coupled with utf-8 encoding settings, fixes the problem.

Signed-off-by: piyo <piyo@users.sf.net>

Tested with (string= current-language-environment "UTF-8") Emacs 24.3.
Sorry no tests because I couldn't pick anything neutral.