Update filter_keys wildcard behaviour to work closer to expectations

[bazerk]

Hey - I just noticed this when I was checking out the library. The current behaviour for filtering of AWS* will filter on any string that contains AWS rather than any string that starts with AWS. I think this is contrary to what most people will expect.

Of course - maybe people are relying on the current behaviour, so maybe you want a documentation change instead to make the current behaviour clearer. Or a config option defaulting how wildcard behaviour is expected to work.

Cross checking against raygun4net and it seems like the python library is lacking a bit of flexibility, the .net library supports startswith*, endswith* and *contains* with what you would expect.