Support Content Attachments API Public Beta

[gmlewis]

GitHub Developer API announcement:
https://developer.github.com/changes/2018-12-10-content-attachments-api/

As far as I can tell, the affected part of the GitHub V3 API is this:
https://developer.github.com/v3/apps/#create-a-content-attachment
but there may be more. Whoever takes this PR should read the announcement carefully and see what all is involved to resolve this issue.

This would be a great PR for any new contributor to this repo or a new Go developer.
All contributions are greatly appreciated!

Feel free to volunteer for any issue and the issue can be assigned to you so that others don't attempt to duplicate the work.

Please check out our CONTRIBUTING.md guide to get started.

Thank you!

[dreamer-89-zz]

@gmlewis : I want to take this issue. Please assign this issue to me.

[waseem18]

Are you working on this @dreamer-89 ? If not I would like to take this up.

[dreamer-89-zz]

Hi @waseem18 , yes I am working on this. This is my first issue, thus struggling with doing this Github App using probot for better understanding. Please give me couple of days time, I will update here on this thread, if i couldn't make any progress.

[waseem18]

Thanks for the prompt reply.

[dreamer-89-zz]

@gmlewis : Please have a look #1084

Right now, it is failing unit test with below two errors. Second one seems to be caused due to first one.

1. CreateAttachment returned error: POST http://127.0.0.1:62948/api-v3/content_references/11/attachments: 404 []
2. CreateAttachment = , want &{ID:0xc000094918 Title:0xc00008b960 Body:0xc00008b970}

Please advise on how to resolve this issue.

[gmlewis]

I commented on the PR.

[dreamer-89-zz]

I commented on the PR.

Thanks a lot @gmlewis for the suggestion. I have updated the PR as per guidelines. Unit tests are working fine now. Please review.

[vaibhavsingh97]

@gmlewis This issue can be closed, as this is fixed in #1139