Use CloseNotifier to supress spurious HTTP 400 errors on early disconnect

When a client disconnects without completing a HTTP request, we were
attempting to process the partial request, which usually leads to a 400
error. These errors can pollute the logs and make it more difficult to
track down real bugs.

This change uses CloseNotifier to detect disconnects. In combination
with checking Content-Length, we can detect a disconnect before sending
the full payload, and avoid logging a 400 error.

This logic is only applied to PUT, POST, and PATCH endpoints, as these
are the places where disconnects during a request are most likely to
happen.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

context/http_test.go

@@ -110,6 +110,13 @@ func (trw *testResponseWriter) Header() http.Header {
 	return trw.header
 }
 
+// CloseNotify is only here to make the testResponseWriter implement the
+// http.CloseNotifier interface, which WithResponseWriter expects to be
+// implemented.
+func (trw *testResponseWriter) CloseNotify() <-chan bool {
+	return nil
+}
+
 func (trw *testResponseWriter) Write(p []byte) (n int, err error) {
 	if trw.status == 0 {
 		trw.status = http.StatusOK