Fix: fail if config.client.args is set to a non-array (e.g an object)

config.client.args was never intended to allow arbitrary objects - the karma code always sets this to an array of strings. This commit ensures that if client.args is set, it's an array.

Adding this check prevents the mess mentioned in https://github.com/karma-runner/karma-mocha/pull/25#issuecomment-35163182