Sequences support Enumerators

This introduces an EnumeratorAdapter so everything coming in looks like
an Enumerator. It doesn't use the adapter if the #peek method exists on
the object.

This allows for sequences like:

    sequence(:cities, %w[Boston Atlanta Detroit Seattle].cycle)

This also makes it easier to cycle through arrays and ranges, since they
can be converted to Enumerators rather easily.

    sequence(:month, (1..12).to_enum)
    sequence(:month, %w[foo bar baz].to_enum)

This doesn't handle when calling Range#step out of the box, because
Ruby returns an Enumerator but ActiveSupport 3.x returns an array,
meaning #to_enum still needs to be called.

Closes #339, #378

lib/factory_girl/sequence.rb

@@ -13,16 +13,36 @@ module FactoryGirl
       options  = args.extract_options!
       @value   = args.first || 1
       @aliases = options.fetch(:aliases) { [] }
+
+      if !@value.respond_to?(:peek)
+        @value = EnumeratorAdapter.new(@value)
+      end
     end
 
     def next
-      @proc ? @proc.call(@value) : @value
+      @proc ? @proc.call(@value.peek) : @value.peek
     ensure
-      @value = @value.next
+      @value.next
     end
 
     def names
       [@name] + @aliases
     end
+
+    private
+
+    class EnumeratorAdapter
+      def initialize(value)
+        @value = value
+      end
+
+      def peek
+        @value
+      end
+
+      def next
+        @value = @value.next
+      end
+    end
   end
 end

spec/factory_girl/sequence_spec.rb

@@ -65,4 +65,14 @@ describe FactoryGirl::Sequence do
       its(:next) { should == "B" }
     end
   end
+
+  describe "iterating over items in an enumerator" do
+    subject { FactoryGirl::Sequence.new(:name, %w[foo bar].to_enum) {|n| "=#{n}" } }
+
+    it "navigates to the next items until no items remain" do
+      subject.next.should == "=foo"
+      subject.next.should == "=bar"
+      expect { subject.next }.to raise_error(StopIteration)
+    end
+  end
 end