[3.7] bpo-34226: fix cgi.parse_multipart without content_length (GH-8530) (GH-20892)

[3.7] bpo-34226: fix cgi.parse_multipart without content_length (GH-8530) (GH-20892)

In Python 3.7 the behavior of parse_multipart changed requiring CONTENT-LENGTH
header, this fix remove this header as required and fix FieldStorage
read_lines_to_outerboundary, by not using limit when it's negative,
since by default it's -1 if not content-length and keeps substracting what
was read from the file object.

Also added a test case for this problem.
(cherry picked from commit d8cf3514dd4682419a66f6e834bb384ee34afc95)

Co-authored-by: roger <rogerduran@gmail.com>
Automerge-Triggered-By: @ned-deily