Fixed parsing of multipart by using parseHeadless

HttpURLConnection's InputStream does not include HTTP header data.
Therefore, the mime stream parser couldn't use the content type
information transported in the content type http header field.

By using parseHeadless instead of parse, the mime parser gets the
content type info by parameter and not from the input stream itself.
Merge pull request #134 from cburkert/fix_mimestream_multipart_parsing

Fixed parsing of multipart by using parseHeadless