Fix errors in ObservableEventsClient caused by incorrect return types

[StanleyGoldman]

#1251 threw me for a loop for a bit.

It turns out this API deals in IssueEvent not Activity.

[StanleyGoldman]

Hmm, I might need some help understanding why the integration tests fail.
I supplied a username, organization, and token in Octokit.Tests.Integration.Helper and the rest seemed to work for me.

[shiftkey]

@StanleyGoldman these failures appear related to the unit tests - check the mocks locally as they may need to be updated.

Also, have you checked the Slow Tests section in the CONTRIBUTING docs? That should help you setup your environment for testing.

[StanleyGoldman]

Hey @shiftkey I didn't see the Slow Tests documentation, I will definitely give it a try tonight.
It's just odd because I did what I feel was an equivalent locally, which passed during tests.
I'll let you know how it goes.

[StanleyGoldman]

@shiftkey Good to go.

[shiftkey]

@StanleyGoldman thanks!