Corrected assertion in zip file export test to match the full content disposition
string, which should include the filename (xform id string).
Changed create_attachments_zipfile() to return a handle to the temporary file-like
object corresponding to the zipped file, rather than the entire data stream of the
file contents, since it is safer for the functions which invoke it to handle it
themselves.

As with the previous revision, this function does *not* leave files in /tmp after
the request is completed.
Updated zip_export() to use the file-like object handle of the zip file, rather than
the full data stream of the prior version.
Updated generate_attachments_zip_export() to use the file-like object handle of the zip file,
rather than the full data stream of the prior version.