fix usage message
mirb dies if #inspect returns a non-string value; fix #2666
mrb_p() should print mrb_obj_as_string() if #inspect does not return a string value
Merge pull request #2667 from murasesyuka/usage_message_is_used_switches_instead_of_option

fix usage message
Fix crash if #inspect does not return a string value

case 1
~~~
class A; def inspect; 1; end; end
A.new.a
~~~

case 2

~~~
class A
  def self.inspect
    1
  end
  alias_method :a, :b
end
~~~
Merge pull request #2668 from sdottaka/fix-crash-for-inspect

Fix crash if #inspect does not return a string value