Fix behaviour of Enumerator#with_index with block

spec/core/enumerator/each_with_index_spec.rb

@@ -0,0 +1,36 @@
+require_relative '../../spec_helper'
+require_relative '../../shared/enumerator/with_index'
+require_relative '../enumerable/shared/enumeratorized'
+
+describe "Enumerator#each_with_index" do
+  it_behaves_like :enum_with_index, :each_with_index
+  it_behaves_like :enumeratorized_with_origin_size, :each_with_index, [1,2,3].select
+
+  it "returns a new Enumerator when no block is given" do
+    enum1 = [1,2,3].select
+    enum2 = enum1.each_with_index
+    enum2.should be_an_instance_of(Enumerator)
+    enum1.should_not == enum2
+  end
+
+  it "raises an ArgumentError if passed extra arguments" do
+    -> do
+      [1].to_enum.each_with_index(:glark)
+    end.should raise_error(ArgumentError)
+  end
+
+  it "passes on the given block's return value" do
+    arr = [1,2,3]
+    arr.delete_if.with_index { |a,b| false }
+    arr.should == [1,2,3]
+  end
+
+  it "returns the iterator's return value" do
+    [1,2,3].select.with_index { |a,b| false }.should == []
+  end
+
+  it "returns the correct value if chained with itself" do
+    [:a].each_with_index.each_with_index.to_a.should == [[[:a,0],0]]
+    [:a].each.with_index.with_index.to_a.should == [[[:a,0],0]]
+  end
+end