resource/github_repository: Allow setting default_branch to master on creation

[ctrom]

Fixes #146.

[ctrom]

@radeksimko could I get a review on this?

[radeksimko]

Hi @ctrom
thanks for the PR.

I have basically two notes/questions:

• Can we instead make this work by calling Update() function right after creation? We do this in many other providers for the same reason (when certain attributes can only be updated, but not passed during creation).
• Can we test this via an acceptance test?

[ctrom]

@radeksimko

• It isn't possible to set a default branch when creating a repo, but regardless of the value that is passed for the default branch, GitHub will set the default branch to master. As a result, I don't think it makes a difference whether we do this on create or update. Really what I want to achieve here is that since GitHub sets the default branch to master when a repo is created, I want the Terraform state to reflect that.
• Perhaps I'm missing it, but I don't see an acceptance test for repo creation. I can attempt to create a new acceptance test for this, but I don't have a lot of experience with golang yet, so please be patient with me while I do.

[ewoutp]

This PR would be a big help.
I'm using github_repository in a module and I right now cannot set the default_branch argument at all.
I like to set it to something like "${var.default_branch}".

This PR would enable me to do so.

Then in the place where I use the module, I could start with default_branch = "master" and later change that to default_branch = "something-else"

[ctrom]

@ewoutp Your situation sounds just like mine. Unfortunately, I don't know enough about golang to finish this quickly and can't take the time right now to learn it. Maybe someone else would be able to take a stab at it?

[tracypholmes]

Hi @ctrom and thanks for the PR! It LGTM but needs a test in order to make sure things work. I hope you don't mind, but I went on and added a test so we can get this merged 😄