test_helpers: Fix POSTRequestMock typing.
test_helpers: Fix POSTRequestMock typing.

The proximal issue here is that in upcoming commits, we're going to
change the type of the `anchor` field in `get_messages_backend` to
support passing either an integer or a string.

Many of our tests using POSTRequestMock currently define a query
object that uses integer values for the integer fields we're going to
pass into it, e.g. {'num_after': 0}.  That is the correct type for
that field in the Zulip API, before HTTP encoding turns it into a
string.  However, because POSTRequestMock didn't use HTTP encoding at
all (which will convert the 0 into a '0'), it ended up passing an
integer to a function that can't possible receive one as an argument.

Ideally, we'd just get rid of POSTRequestMock, since it's a hack, and
just do real HTTP requests instead.

But since it's used in a lot of places making doing so somewhat
impractical, we can get past this issue by just making POSTRequestMock
convert integers to strings.

zulip/zulip,synicalsyntax/zulip,zulip/zulip,punchagan/zulip,brainwane/zulip,andersk/zulip,synicalsyntax/zulip,timabbott/zulip,timabbott/zulip,andersk/zulip,andersk/zulip,punchagan/zulip,shubhamdhama/zulip,showell/zulip,andersk/zulip,punchagan/zulip,kou/zulip,zulip/zulip,showell/zulip,rht/zulip,eeshangarg/zulip,brainwane/zulip,showell/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,shubhamdhama/zulip,kou/zulip,brainwane/zulip,shubhamdhama/zulip,punchagan/zulip,showell/zulip,kou/zulip,rht/zulip,timabbott/zulip,zulip/zulip,rht/zulip,brainwane/zulip,shubhamdhama/zulip,synicalsyntax/zulip,eeshangarg/zulip,synicalsyntax/zulip,rht/zulip,punchagan/zulip,synicalsyntax/zulip,zulip/zulip,showell/zulip,timabbott/zulip,timabbott/zulip,andersk/zulip,kou/zulip,brainwane/zulip,zulip/zulip,shubhamdhama/zulip,rht/zulip,eeshangarg/zulip,showell/zulip,kou/zulip,shubhamdhama/zulip,brainwane/zulip,zulip/zulip,brainwane/zulip,showell/zulip,synicalsyntax/zulip,timabbott/zulip,hackerkid/zulip,eeshangarg/zulip,timabbott/zulip,punchagan/zulip,hackerkid/zulip,hackerkid/zulip,shubhamdhama/zulip,rht/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,punchagan/zulip,synicalsyntax/zulip