api_call method returning string, not dict about python-slack-sdk

[rawdigits]

Sorry about that! It appears I always return a string, and was incorrect in the docs. I'll update the docs.

[elementc]

I've provided PRs #70 and #71 to fix this issue. #70 fixes the api_call func to do a json.loads() before returning- I prefer this solution to #71's since it's more consistent with rtm_read()'s returning of dicts. #71 only fixes the docs to show api calls being passed through json.loads(). Take your pick which you merge.

[elementc]

#70 was obsoleted by #74 which also fixes #73 in addition to this issue.

[ravenCondol]

The examples in the docstring for api_call still say that it returns a string rather than a dict. Minor thing, but thought I'd say in case someone else stumbles on this ahaha