Fixes failed test.

Accordingly to http://www.faqs.org/rfcs/rfc2047.html and the right implementation in https://github.com/zendframework/zf1/blob/master/library/Zend/Mime.php on line 196, commas have to be written as =2C which is their ascii representation.
Merge pull request #143 from siad007/patch-1

Fixes failed test in Zend_Mime::encodeQuotedPrintableHeader()