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.