conformance: Connect GET requests should not allow a body or content-headers

[jhump]

Since GET requests do not have a body -- request data is instead encoded in the query string -- the client should not send content headers (such as content-length, content-type, content-encoding). Server should not allow a GET requests that does include a non-empty body.

The client in this repo is including content-type headers and the server is not confirming that the body is empty for GET requests.

Found by conformance tests in v1.12.0.

[jhump]

This bug was fixed by #644, and the fix was released in v1.13.0.