Flag the really_responds_to? fallback as nocov

In contexts without RSpec::Mocks::Double defined, we don't need to check this, but of course it's always defined in our _rspec tests._

Author: nevinera

lib/rspec/matchers/built_in/has.rb

@@ -58,9 +58,11 @@ def really_responds_to?(method)
  end
  end
  else
+ # :nocov:
  def really_responds_to?(method)
  @actual.respond_to?(method)
  end
+ # :nocov:
  end
 
  def predicate_accessible?