Add optional(false) support to belong_to

[guialbuk]

Allows belongs_to associations with optional: false

belongs_to :copany, optional: false

to be tested with

it { should belong_to(:copany).optional(false) }

It also adds support for optional(true) which is the same as just optional.

Fixes #1219

[mcmire]

Oh nice! I just did a quick pass of this and it looks good but I will take a closer look soon.

[bbugh]

Hurray! 🎉 Our app has belongs_to_required_by_default = false and only sets optional: false when required, which means there's no way for us currently to test this with shoulda-matchers. We need to explicitly test optional(false). Thanks for doing this PR!

[mcmire]

Okay, looks good! Merge at will 👍

[XavierMeunier]

Thanks for your PR
I'm also waiting for this feature to be released, is there any planned date for the release of 4.2 ?

This will also be usefull for conditional optional in Rails 5:
belongs_to :category, optional: -> { draft? }