the behavior of each_index was incorrect

when run under an array, the behavior is different than the enumerable
version

ruby/lib/google/protobuf/repeated_field.rb

@@ -70,7 +70,7 @@ class RepeatedField
       def_delegators :to_ary,
         :&, :*, :-, :'<=>',
         :assoc, :bsearch, :combination, :compact, :count, :cycle,
-        :drop, :drop_while, :eql?, :fetch, :find_index, :flatten,
+        :drop, :drop_while, :each_index, :eql?, :fetch, :find_index, :flatten,
         :include?, :index, :inspect, :join,
         :pack, :permutation, :product, :pretty_print, :pretty_print_cycle,
         :rassoc, :repeated_combination, :repeated_permutation, :reverse,
@@ -104,7 +104,6 @@ def empty?
       end
 
       # array aliases into enumerable
-      alias_method :each_index, :each_with_index
       alias_method :slice, :[]
       alias_method :values_at, :select
       alias_method :map, :collect

ruby/tests/repeated_field_test.rb

@@ -371,6 +371,20 @@ def test_delete_at
     end
   end
 
+  def test_each_index
+    m = TestMsg::Message.new
+    reference_arr = %w(foo bar baz)
+    m.repeated_string += reference_arr.clone
+    check_self_modifying_method(m.repeated_string, reference_arr) do |arr|
+      result = []
+      arr.each_index {|x| result << [x, " -- "] }
+      result
+    end
+    check_self_modifying_method(m.repeated_string, reference_arr) do |arr|
+      arr.each_index
+    end
+  end
+
   def test_fill
     m = TestMsg::Message.new
     reference_arr = %w(foo bar baz)