riakc_obj:new spec type is incorrect [JIRA: CLIENTS-907]

[gleber]

Current it is defined as:

-spec new(bucket(), key(), value(), content_type()) -> riakc_obj().

while code suggests it should be

-spec new(bucket(), key(), value(), content_type()) -> riakc_obj() | {error, atom()}.

[gleber]

I am not entirely sure why this happens, but there's also the following effect:

The variable Obj can never match since previous clauses completely covered the type {'error','zero_length_bucket' | 'zero_length_key'}

reported for the following code:

case riakc_obj:new(<<"test">>, <<"foo">>, <<"bar">>) of
    {error, Error} -> erlang:error({error, Error});
    Obj -> io:fwrite("~p~n", [Obj])
end

Somehow Dialyzer of OTP 19.0.1 thinks that riakc_obj:new/3 can only return error tuples, but not riakc_obj()

[lukebakken]

There seems to be dialyzer issue with OTP 19. Going to move this to 2.5.0 for now. Thanks for the report ... and a PR would be welcome if you have time 🍥