[Test-Proxy] Improve playback matching algorithm

When a match is not found for an incoming request, we report back the closest match and detail the differences in the exception. Currently, the algorithm uses the entry that had the fewest differences as the closest match, where a difference can be in a header, URI, method, etc. We should probably give precedence to matching methods, and possibly even URIs.

This came up in a Storage test where a HEAD request was missing several headers as compared to the entry record. The Test Proxy reported back that there was a mismatch between the HEAD request and a DELETE request, simply because the DELETE request had fewer differences. It would be much better for troubleshooting if the exception chose the HEAD request and reported back the missing headers.