Add doctest to the test template

Signed-off-by: Mikhail S. Pobolovets <styx.mp@gmail.com>
Merge pull request #2889 from styx/add_doctest_to_mix_new_test_template

[MIX] Add doctest to the test template
Make map_reduce @spec stricter

Signed-off-by: Mikhail S. Pobolovets <styx.mp@gmail.com>
Merge pull request #2890 from styx/more_strict_spec_for_map_reduce

Make map_reduce @spec stricter
Merge branch 'master' of https://github.com/elixir-lang/elixir