prevent "308 PERMANENT REDIRECT" via client change

[melange396]

Recent changes to the python client in #1288 make it so requests are made to endpoints directly, instead of using the api.php alias to do the proper routing. However, the client is not currently appending a trailing slash to these URLs, which prompts a "308 PERMANENT REDIRECT" response from the server.

The client "correctly" follows this up by re-requesting with the trailing slash present, but this wastes time on the client side by doubling the number of requests made, and wastes cycles on the server side.

compare for yourself the difference in the results from the two commands:
curl -vvvv https://api.delphi.cmu.edu/epidata/covidcast
and
curl -vvvv https://api.delphi.cmu.edu/epidata/covidcast/

Once this fix is complete, it should also automatically get applied to the covidcast client as well, as soon as users of that package update their dependencies to include the appropriate delphi-epidata release.