Incosistency on Connection.offset/1

[ggcampinho]

Current behavior

When the pagination is :forward the offset is changed, but when the pagination is :backward the offset remains the same of the request. Ex:

iex(2)> Connection.offset(%{first: 2, after: Connection.offset_to_cursor(2)})
3
iex(3)> forward_args = %{first: 2, after: Connection.offset_to_cursor(2)}
%{after: "YXJyYXljb25uZWN0aW9uOjI=", first: 2}
iex(4)> Connection.offset(forward_args)
3
iex(5)> Connection.limit(forward_args) 
{:ok, :forward, 2}
iex(6)> backward_args = %{last: 2, before: Connection.offset_to_cursor(2)}
%{before: "YXJyYXljb25uZWN0aW9uOjI=", last: 2}
iex(7)> Connection.offset(backward_args)                                  
2
iex(8)> Connection.limit(backward_args) 
{:ok, :backward, 2}

Expected behavior

iex(2)> Connection.offset(%{first: 2, after: Connection.offset_to_cursor(2)})
3
iex(3)> forward_args = %{first: 2, after: Connection.offset_to_cursor(2)}
%{after: "YXJyYXljb25uZWN0aW9uOjI=", first: 2}
iex(4)> Connection.offset(forward_args)
3
iex(5)> Connection.limit(forward_args) 
{:ok, :forward, 2}
iex(6)> backward_args = %{last: 2, before: Connection.offset_to_cursor(2)}
%{before: "YXJyYXljb25uZWN0aW9uOjI=", last: 2}
iex(7)> Connection.offset(backward_args)                                  
0
iex(8)> Connection.limit(backward_args) 
{:ok, :backward, 2}

Internally the calculation actually happens at https://github.com/absinthe-graphql/absinthe_relay/blob/master/lib/absinthe/relay/connection.ex#L279, but it seems to me that the method is behaving differently depending on the direction and would be easier if it returns the proper offset for the query.

The documentation on the method also explains that this is the expected behavior, I can submit a PR if that makes sense