Handle multiple Content-Length headers with the same value #92
Description
From RFC 7230 Section 3.3.2:
If a message is received that has multiple Content-Length header
fields with field-values consisting of the same decimal value, or a
single Content-Length header field with a field value containing a
list of identical decimal values (e.g., "Content-Length: 42, 42"),
indicating that duplicate Content-Length header fields have been
generated or combined by an upstream message processor, then the
recipient MUST either reject the message as invalid or replace the
duplicated field-values with a single valid Content-Length field
containing that decimal value prior to determining the message body
length or forwarding the message.
Currently h11 will error out if multiple Content-Length headers are sent even when equal in value. Perhaps we should collapse the multiple headers into only one or allow the duplicate to pass through if the values are the same?