Fix delegate_method when used with shoulda-context

This is a fix for #581.

----

When specifying a custom subject, a straightforward usage of
`delegate_method`, such as the following

    class UserPresenter
      delegate :id, to: :user

      attr_reader :user

      def initialize(user)
        @user = user
      end
    end

    class UserPresenterTest < ActiveSupport::TestCase
      subject { UserPresenter.new(User.new) }
      should delegate_method(:id).to(:user)
    end

would error with something like:

    ArgumentError: wrong number of arguments (0 for 1)

This happens because the `should` method in shoulda-context asks the
DelegateMethodMatcher object for its description so that it can use it
to give the test a name and create a method that Test::Unit can run.
Now, the matcher's description needs a subject in order to determine
whether a class or an instance is being tested here -- if a class is
being tested the description will be "should delegate #id to \#user
object", if a class then "should delegate .id to .user object".
Unfortunately the matcher doesn't know what the subject is before
its #description method is called -- it only knows about this when it
gets evaluated.

Within the matcher we do have access to the current context class, so we
could read the subject block off of it and evaluate it. However, in
order to properly do this we also need access to the instance of the
test itself, which we do not have until the matcher is evaluated (by
which point it's too late).

Since there's really no way to solve this problem apart from rewriting a
lot of shoulda-context, and since often times your subject is an
instance and not a class, just assume it's an instance in this case.