Handle union case for expected value

When using both forms of the HaveMatcher - expected can be a Char,
String, or Regex. Regex is invalid for String#includes?.
I believe this might be a generics issue that was fixed recently in
Crystal.

src/spectator/matchers/have_matcher.cr

@@ -19,6 +19,7 @@ module Spectator::Matchers
     private def match_string?(actual_value)
       expected.value.all? do |item|
         actual_value.includes?(item)
+        actual_value.includes?(item) if item.is_a?(Char | String)
       end
     end