Style/RaiseArgs doesn't handle exception classes with just keyword arguments properly

[wvanbergen]

Expected behavior

When using an exception class with just keyword arguments, it should not find an offense.

raise MyError.new(keyword: argument, key: word)

Actual behavior

It finds an offense, telling me I should be using raise MyError, message instead. This is because the code simply checks if there's only 1 argument, and keyword argument always count as one.

Steps to reproduce the problem

Run rubocop on the following snippet, with Style/RaiseArgs enabled

raise MyError.new(keyword: argument, key: word)

RuboCop version

Include the output of rubocop -V. Here's an example:

rubocop -V
0.46.0 (using Parser 2.3.3.1, running on ruby 2.3.3 x86_64-darwin16)

[mikegee]

I confirmed the reported behavior on master with the default configuration. Additionally, raise MyError.new(argument, word) produces no offense.

This seems like an inconsistency that should be allowed.