Add support for required keyword arguments

Required keyword arguments were being collected under the `ParameterNode#optional_params`, but was causing an error as the default argument part of the s-expr was `false`.

I've solved this by rejecting any entry that has the default set to `false`, and added the params with default set to `false` to `required_params`.

Specs included, and a quick test by using `@param keyword_arg: [Type] Message` appears to work correctly.