bpo-34226: fix cgi.parse_multipart without content_length (GH-8530)

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 d8cf3514dd)

Co-authored-by: roger <rogerduran@gmail.com>

Lib/test/test_cgi.py

@@ -128,6 +128,20 @@ class CgiTests(unittest.TestCase):
                     'file': [b'Testing 123.\n'], 'title': ['']}
         self.assertEqual(result, expected)
 
+    def test_parse_multipart_without_content_length(self):
+        POSTDATA = '''--JfISa01
+Content-Disposition: form-data; name="submit-name"
+
+just a string
+
+--JfISa01--
+'''
+        fp = BytesIO(POSTDATA.encode('latin1'))
+        env = {'boundary': 'JfISa01'.encode('latin1')}
+        result = cgi.parse_multipart(fp, env)
+        expected = {'submit-name': ['just a string\n']}
+        self.assertEqual(result, expected)
+
     def test_parse_multipart_invalid_encoding(self):
         BOUNDARY = "JfISa01"
         POSTDATA = """--JfISa01