Custom snapshot name isn't respected

[aaronpowell]

Describe the bug
This is related to #178, as I was trying to work around the problem there by providing a custom name for the snapshot, but I found that even if you do provide a custom name the resolver is still called (where the bug in #178 is), nullifying the workaround. The offending line is here:

https://github.com/SwissLife-OSS/snapshooter/blob/master/src/Snapshooter/SnapshotFullNameResolver.cs#L69

To Reproduce

• Follow the steps in MS Test data driven tests fail if data param contains null #178 but provide a custom name to Snapshot.Match.
• See test fail as the MSTestSnapshotFullNameReader was still executed

Expected behavior
The custom name to be used and no logic to attempt name generation is executed.