Do not split long boundary

[Fneufneu]

long boundary are currently split and this break messages.
I known RFC 2046 does not allow boundary to be more than 70 char but here is an exemple with 66 chars.

A message header with

Content-Type: multipart/mixed; boundary="----=_=-_OpenGroupware_org_NGMime-26739-1764058188.613387-49------"

become

Content-Type: multipart/mixed;
 boundary*0="----=_=-_OpenGroupware_org_NGMime-26739-1764058188.613387-49----";
 boundary*1=--

And the mail become invalid.

With this patch:

Content-Type: multipart/mixed;
 boundary="----=_=-_OpenGroupware_org_NGMime-26739-1764058188.613387-49------"