Change how values are passed to helper

Working around a segfault (dunno why this fixes it).

src/spectator/matchers/generic_match_data_value.cr

@@ -3,6 +3,9 @@ require "./match_data_value"
 module Spectator::Matchers
   # Wraps a value for used in match data.
   private class GenericMatchDataValue(T) < MatchDataValue
+    # Underlying value.
+    getter value
+
     # Creates the wrapper.
     def initialize(@value : T)
     end