Rename test method...

...as per review suggestion https://github.com/jclouds/jclouds/pull/120/files#r5909436
ETagUtils.unquote(): Refactor to use a regex...

...as per review comments https://github.com/jclouds/jclouds/pull/120/files#r5909783 and https://github.com/jclouds/jclouds/pull/120/files#r5914046
Add @VisibleForTesting annotation to explain package level visibility
Rename method and add comment to make it clearer that it only accepts hex ETags, not arbitrary ETags