With the deprecation of "should_allow_values_for" there is currently no clean
way to specify multiple allowed (or forbidden) values for an attribute in a
single assertion. This can make test cases unnecessarily verbose.

This patch gives AllowValueMatcher the ability to take multiple values. The new
matcher is fully downward compatible (i.e. given a single argument it behaves
identically to the old matcher).

This matcher will stop checking for bad values when the first bad value is
encountered.

…to shoulda-matchers

In this merge, we also adjust the spec to RSpec syntax.

Conflicts:
	spec/shoulda/active_model/allow_value_matcher_spec.rb