http_proxy: Ignore TE and CL in CONNECT 2xx responses

A client MUST ignore any Content-Length or Transfer-Encoding header
fields received in a successful response to CONNECT.
"Successful" described as: 2xx (Successful). RFC 7231 4.3.6

Prior to this change such a case would cause an error.

In some ways this bug appears to be a regression since c50b878c. Prior to
that libcurl may have appeared to function correctly in such cases by
acting on those headers instead of causing an error. But that behavior
was also incorrect.

Bug: https://github.com/curl/curl/issues/1317


Reported-by:  <mkzero@users.noreply.github.com>