89948, Remove warning when Content-Id is not present

According to http://www.w3.org/TR/SOAP-attachments

> Referenced MIME parts must contain either a Content-ID MIME header
> structured in accordance with RFC 2045, or a Content-Location
> MIME header structured in accordance with RFC 2557

Content-Id is not mandatory and can be undefined.

Fix warning produced after applying regexp to an undefined value of
Content-Id in such cases.
Merge pull request #6 from elvenfighter/fix_content_id_89948

89948, Remove warning when Content-Id is not present