Fix conditional validator evaluation for Class subjects and lambda arity

[johnnyshields]

The 4.2.0 conditional validator support (commit bd9577e) was added with the intention of checking conditional validations on models.

This change was well-intentioned, but it was done in a way that broke semver. Specifically, it enforced conditional checks on all validation cases in a way that would cause specs which were passing on 4.1.0 to now fail.

To fix the fundamental semver issue, this PR puts the "conditional validator support" logic behind a gate method .check_conditions:

subject.validates_presence_of(:foobar).check_conditions

In addition, bd9577e had 3 subtler issues which are fixed in this PR:

1. It introduced a hard error on Class subjects, which broke semver. This PR raises an informative error if you use .check_conditions on a Class subject (which one wouldn't do by default; it would be a clear sign of a miswritten spec).
2. instance_exec broke lambdas with explicit args (->(obj) { ... }). This PR now uses filter.call(actual) for non-zero arity.
3. It had no support for Array conditions; Rails wraps if/unless into arrays when using on:/except_on: options

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[johnnyshields]

@dblock this PR is ready for your review

[johnnyshields]

@dblock second thoughts on this. I think we should make a new modifier .with_conditions (or .check_conditions!) that explicitly enables the instance-level check.

expect(subject).to validate_presence_of(:name).with_conditions

The way this feature was implemented in mongoid-rspec 4.2.0 broke a bunch of my cases that were working fine in 4.1.0.

[dblock]

@johnnyshields feel free to merge as is or wait and implement your suggestion

[johnnyshields]

OK, let me have a think, I will probably do a re-implementation here later today.

[johnnyshields]

@dblock I've gone ahead and merged this. In a nutshell, I've gated the "conditional valdiation" feature introduced in 4.2.0 (PR #239) behind a .check_conditions method, which preserves semver compatibility but allows users who want the feature to opt-into it.

To use it:

it "validates foobar field conditionally" do
  subject.validates_presence_of(:foobar).check_conditions
end

We could consider a more substantial breaking change in mongoid-rspec 5.0 if users really want this behavior, or perhaps a global flag such as Mongoid::Rspec.default_check_conditions = true. Personally, I think it is probably the wrong abstraction; .check_conditions feels a bit magical to me and I prefer to write more explicit RSpec cases to check validation conditions.

@knovoselic please note, you will need to update your tests to use the .check_conditions method where you want to use this.

[dblock]

I think a major version bump and removing the magic makes sense.