Disable negated matcher for RSpec

[coorasse]

Let's not allow to specify expect(...).not_to be_able_to(:action_a, :action_b) for multiple actions. This lead in the past to too many open issues. Here are some:

• negated matcher for be_able_to only passes when no ability is eligible #646
• Implement negative expectation handler logic for be_able_to matcher #602
• Negated matcher of "be_able_to" has unexpected behavior #505
• CanCan/matchers doesn't work when multiple permissions are checked at the same time #362

Here is a recap: Marc is allowed to eat apples but not bananas.

can :collect, 🍎 
cannot :eat, 🍎 

expect(mark).to be_able to(:collect, 🍎 )
expect(mark).not_to be_able to(:eat, 🍎 )

# this below will fail, because Mark cannot eat 🍎 
expect(mark).to be_able to([:eat, :collect], 🍎 ) 

# this below will pass, because is exactly the opposite of the one above.
expect(mark).not_to be_able to([:eat, :collect], 🍎 ) 

but a reader might say "I expect Mark to not be able to eat the 🍎 !", why is he able to? This caused quite some confusion and nasty bugs. From now on you have to split the one above into:

expect(mark).to be_able to(:collect, 🍎 ) 
expect(mark).not_to be_able to(:eat, 🍎 )