Spec: cast values for be_nil and be_a expectations

[straight-shoota]

A way to solve this forum request

Currently, when validating the type of a value in spec, we need to restrict the type afterwards in order to call restricted methods on the sample:

index = "foo".index('o')
index.should_not be_nil
foo[index.not_nil!].should eq('o')

The not_nil! is needed because the compiler has no way to know that the should_not be_nil means that index will not be nil.

The idea is to let be_nil and be_a expectations return the value with appropriate type restriction:

index = "foo".index('o')
index = index.should_not be_nil
foo[index].should eq('o')

Basically, the result of obj.should_not be_nil will return obj as a non-nil object.

The same applies for obj.should be_a(T) and obj.should_not be_a(T).

An implementation was submitted in #8240, but it should be overhauled.