Handle Regexp like String when apportioning args

If the first argument is a Regexp, it's a message _matcher_, rather than
a message.. but certainly it's not an exception! I'm actually a bit
surprised this doesn't cause any other problems, but it doesn't seem to.

lib/rspec/matchers/built_in/raise_error.rb

@@ -25,7 +25,7 @@ def initialize(expected_error_or_message, expected_message, &block)
           when nil, UndefinedValue
             @expected_error = Exception
             @expected_message = expected_message
-          when String
+          when String, Regexp
             @expected_error = Exception
             @expected_message = expected_error_or_message
           else