Create tests for `createIssue()` method

[jfrerich]

Summary

There currently aren't any test for the GitHub plugin api layer. This request is to add test for the createIssue() method found in server/api.go

Is there a reasonable way for us to add tests for this kind of functionality in the GitHub plugin?

I don't see a reason why we cannot add tests for this functionality. Currently, there are no tests for the api layer. I will create as another ticket and add to the 0.15.0 milestone for tracking. There are 5 other tickets for this feature and I am going to work on those next. This was the first of 6.

Originally posted by @jfrerich in https://github.com/mattermost/mattermost-plugin-github/pull/222#issuecomment-603002586