Fix bug in calculation of received headers

This would manifest itself is the headers were sent in multiple
individual packets that were processed after each other, the calculation
for max header size would be wrong and Waitress would incorrectly return
RequestHeaderFieldsTooLarge.

src/waitress/parser.py

@@ -103,7 +103,7 @@ def received(self, data):
                 # If the headers have ended, and we also have part of the body
                 # message in data we still want to validate we aren't going
                 # over our limit for received headers.
-                self.header_bytes_received += index
+                self.header_bytes_received = index
                 consumed = datalen - (len(s) - index)
             else:
                 self.header_bytes_received += datalen