Trim header values before they are being set in the headers_holder

Some email clients send the Content-Type header of an attachment like this:

Content-Type:
    application/octet-stream; name="consoletools-table.png"

If the header value is not trimmed, the Content-Type value has a space at the beginning which leads to the regular expression in ezcMailPartParser::createPartParserForHeaders not matching. Because of this the attachments are not correctly parsed.