SpecifierSet.filter should allow pre-release when no final version matches specifiers, same as Specifier.filter

[notatallshaw]

Fixes #856

This brings SpecifierSet in-line with Specifier and the spec. I believe this behavior was incorrectly created from #29, which determined ahead of filtering whether to allow pre-releases or not, but did not introduce a fallback when only pre-releases matched the specifiers.

Interestingly, even though the old behavior is listed in the doc string there were previously no tests for this behavior:

>>> list(SpecifierSet(">=1.2.3").filter(["1.2", "1.5a1"]))
[]

The docstring has been updated and the equivalent tests are added.

[notatallshaw]

FWIW, I prefer writing a method to return early, and not have large else blocks, but this PR focuses on fixing the bug without refactoring or writing code that conflicts with the existing style.