Added dig and bsearch_index to RepeatedField methods forwarded to array
Added dig and bsearch_index to RepeatedField methods forwarded to array

This fixes the test_acts_likes_an_array test in RepeatedFieldTest, which
checks that repeated fields respond to the same methods as regular Ruby
arrays. The bsearch_index and dig array methods seem to be new in Ruby
2.3 and so we should support those.

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