No fake filters in fingerprints

When Brakeman encounters a `before_filter` using a block, it generates a new method with a random name to contain the filter. This makes processing easier. But when the warning is reported, the random name is used in the fingerprint and as the method name. Since the name will change each time, this means the fingerprint will also change.

Instead, with this change the method name will always be reported as `before_filter`.