Varying boundary length

At Swift_Mime_Headers_ParameterizedHeader::_createParameter boundary can be encoded because the length is bigger then allowed by $_maxLineLength.

Both the getmypid() and mt_rand() function returns varying length result, anyway I don't know if pid in boundary is a good idea.

Maybe using md5 is better way to limit the max length and getting bigger entropy... 

Thx @smatyas for the help.