diffHttpRequest should validate headers if specified in expectedRequest

[nealeu]

It looks like this was intended but never implemented.

How about validating HTTP headers if they have been specified in expectedRequest?

I think serialising the headers as Json and then using the Json diff would be the most user-friendly.

I'd be happy to provide a PR. Any reason not to?

e.g. this does the job:

	diffStrings(t, fmt.Sprintf("%s.uri", description), actual.Uri, expected.Uri)
	if expected.Headers != nil {
		actualHeader, _ := json.Marshal(actual.Headers);
		expectedHeader, _ := json.Marshal(expected.Headers);
		diffJson(t, description, actualHeader, expectedHeader)
	}
	diffJson(t, description, actual.Body, expected.Body)