Fix logic bug - add_offset is in encoding, not attrs.

[mankoff]

_pop_to does a pop operation - it removes the key/value pair. So the line above this change will remove add_offset from attrs if it exists. The second line then checks for add_offset in attrs which should always be False.

This is implemented in #6812 but the work there is significantly more involved (see for example different PR addressing the same problem, #2304), so I'm pulling this fix out and submitting it as its own PR.

[dcherian]

Thanks @mankoff Is there a test we could add?

[mankoff]

Thanks @mankoff Is there a test we could add?

There's a whole table of tests! #2304 (comment)

But now I'm building a test for the code as-is, which isn't CF-compliant. Is this worth writing?

[dcherian]

ah let's just add that to the other PR.

Thanks! This is a great contribution