Don't allow mappings to be passed to integer_type:

This was a relic of development, when we couldn't pass the mapping directly to `polymorphic:`

Author: jnraine

lib/polymorphic_integer_type/extensions.rb

@@ -13,7 +13,7 @@ def belongs_to(name, scope = nil, options = {})
             when true then PolymorphicIntegerType::Mapping[name]
             when nil then options[:polymorphic]
             else
-              integer_type
+              raise ArgumentError, "Unknown integer_type value: #{integer_type.inspect}"
             end.dup
 
           foreign_type = reflections[name.to_s].foreign_type

spec/polymorphic_integer_type_spec.rb

@@ -154,8 +154,8 @@ class InlineLink < ActiveRecord::Base
 
       self.table_name = "links"
 
-      belongs_to :source, polymorphic: true, integer_type: {10 => "Person", 11 => "InlineAnimal"}
-      belongs_to :target, polymorphic: true, integer_type: {10 => "Food", 13 => "InlineDrink"}
+      belongs_to :source, polymorphic: {10 => "Person", 11 => "InlineAnimal"}
+      belongs_to :target, polymorphic: {10 => "Food", 13 => "InlineDrink"}
       belongs_to :normal_target, polymorphic: true
     end