Amend :length validation inference to work on any derivation of Text …

…or String.

Previously, inferred validation for :length only worked on Text or String
property classes only.  But if one of these types is subclassed, the common
expectation is more likely to be that their (auto) validations are inherited as
well as their behaviours.

lib/dm-validations/auto_validate.rb

@@ -130,7 +130,7 @@ def infer_presence_validation_for(property, options)
       end
 
       def infer_length_validation_for(property, options)
-        return unless [ DataMapper::Property::String, DataMapper::Property::Text ].include?(property.class)
+        return unless [ DataMapper::Property::String, DataMapper::Property::Text ].any? { |klass| property.kind_of?(klass) }
 
         case length = property.options.fetch(:length, DataMapper::Property::String::DEFAULT_LENGTH)
           when Range then options[:within]  = length