Added anchor to getPrettyDomain regex

I added an anchor to the regex in Lib.getPrettyDomain. As it was, it was unanchored.

I suspect this was probably a bug anyway (I don't know of a use case where there's a domain, but it isn't at the start of the URL), but it also had the side effect of making HAR files with long `data:` URLs slow to process.

Anchoring the regex solves both issues.

The test pack has been run against the changed code, and passes.