Make switch_option return a strict boolean

This is so that the `current_is_option?` method properly matches its own
documentation, and consistently returns a pair of booleans.

Author: deivid-rodriguez

lib/thor/parser/options.rb

@@ -171,7 +171,7 @@ def current_is_value?
     end
 
     def switch?(arg)
-      switch_option(normalize_switch(arg))
+      !switch_option(normalize_switch(arg)).nil?
     end
 
     def switch_option(arg)