2to3: use Request.get_data() instead of Request.data

This is the right thing to do according to
pallets/flask#716.
— data is intentionally the undecoded bytes, and we need to either
decode it outselves, our use .get_data(as_text=True). This latter
seems to be a nicer option.

I used temporary 'data' variable to assign the decoded text.
.get_data() is caching by default, so this doesn't save much work,
but avoid the fairly verbose method invocation in asserts.

In some places 'data' was used for .post(), and this is renamed
to 'post_data' to avoid the name clash.

This makes all tests pass under python3.

Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>