Use get_args to get next link

[babolivier]

Turns out setting a next link when validating a 3PID is broken since the Python 3 work. Briefly, request.args stores its keys as bytes, so 'nextLink' in request.args works in Python 2 but not in Python 3.

The fix for this is to use get_args, which does the right thing in that regards, and has support for optional arguments since #294.

I assume I missed this when adding Python 3 compatibility because I was assuming get_args was the only way Sydent extracts arguments from a requests - turns out I missed that bit which was doing it another way. There doesn't seem to be other occurrences of that bug (looks like the only occurrence of request.args in the whole codebase is in get_args).

[clokep]

I assume it is OK to use get_args here even though that pulls out from the JSON in some situations and such?

[clokep]

Seems OK. Thanks for checking there were no other occurrences!

[babolivier]

I assume it is OK to use get_args here even though that pulls out from the JSON in some situations and such?

Should be - get_args is a catch-all sort of function that grabs every argument a request might include, either in the URL or in the JSON or both.