Fix "warning: deprecated Object#=~ is called on Integer"

`Object#=~` is deprecated in Ruby 2.6.
Ref: https://svn.ruby-lang.org/cgi-bin/viewvc.cgi?revision=65989&view=revision

Author: y-yagi

lib/thor/parser/arguments.rb

@@ -9,7 +9,7 @@ def self.split(args)
       arguments = []
 
       args.each do |item|
-        break if item =~ /^-/
+        break if item.is_a?(String) && item =~ /^-/
         arguments << item
       end