This makes the tests easier to copy and relocate to other test numbers without having to update content. Closes #6738
... so that we can check HTTP requests for User-Agent: curl/%VERSION Update 600+ test cases accordingly. Closes #6037
With CURLOPT_TIMECONDITION set, a header is automatically added (e.g. If-Modified-Since). Allow this to be replaced or suppressed with CURLOPT_HTTPHEADER. Fixes #4103 Closes #4109