Fix `ArgumentError` when there are no keyword arguments (Ruby < 2.7)

[sylvain-8422]

Calling helper methods that are not meant to receive keyword arguments fails with RABL version 0.16.0 and using Ruby < 2.7.

Without the fix, the two new assertions fail.

Here a simple IRB session demonstrating the source of the problem (using Ruby 2.6.10) :

object = ::Object.new

def object.method
  :value
end

object.__send__(:method, **{})
=> :value

kwargs = {}

object.__send__(:method, **kwargs)
ArgumentError (wrong number of arguments (given 1, expected 0))

The fix would be much appreciated, as otherwise we would have to restrict RABL to < 0.16.0.

[nesquena]

Thanks!

[nesquena]

Pushed as rabl 0.16.1, thanks again!