Ensure standalone inclusions don't clear exclusions

Fixes #1484

Standalone inclusions still need to allow for exclusions
Move if/unless specs to a more appropriate location

The Configuration specs were too closely tied to
the implementation of the filter manager, so this
step is allow for changes in the underlying impl
without losing coverage of the expected behavior.
Combine inclusion/exclusion filter shared behavior
Treat if/unless as unchangeable behavior of ExclusionFilter
Add feature covering if/unless always working
Add entry describing if/unless regression fix
Merge pull request #1498 from rspec/fix-exclusions-regression

Ensure standalone inclusions don't clear exclusions