[#79] missing spec added

[ferigis]

[close #79]

[cabol]

I'd add some floats and atoms

[cabol]

I recommend to add a nested/complex structure, like:

-spec types(sr_test_utils:config()) -> {comment, string()}.
types(_Config) ->
   Nested = #{a => #{b => [1, #{c => 3}]}},
  JsonTypes = [ [<<"binary">>, <<"binary">>, <<"binary">>]
              , [1, 2, 3, 4, 5]
              , [#{a => 1}, #{b => 2}]
              , Nested
              , [Nested, <<"mixed">>, <<"list">>, 3, #{a => []}]
              ],
  _ = [sr_json:encode(Json) || Json <- JsonTypes],

  {comment, ""}.