Fix #207; camelize strings instead of classifying them

Before the model filter would turn "things" into "Thing". Now it turns
"things" into "Things".

lib/active_interaction/filters/model_filter.rb

@@ -39,7 +39,7 @@ def cast(value, reconstantize = true)
     #
     # @raise [InvalidClassError]
     def klass
-      klass_name = options.fetch(:class, name).to_s.classify
+      klass_name = options.fetch(:class, name).to_s.camelize
       Object.const_get(klass_name)
     rescue NameError
       raise InvalidClassError, klass_name.inspect

spec/active_interaction/filters/model_filter_spec.rb

@@ -3,6 +3,7 @@
 require 'spec_helper'
 
 class Model; end
+class Things; end
 
 describe ActiveInteraction::ModelFilter, :filter do
   include_context 'filters'
@@ -118,6 +119,16 @@ def self.name
       end
     end
 
+    context 'with a plural class' do
+      let(:value) { Things.new }
+
+      before { options[:class] = Things }
+
+      it 'returns the instance' do
+        expect(result).to eql value
+      end
+    end
+
     context 'with class as an invalid String' do
       before do
         options.merge!(class: 'invalid')