Deprecate Pathname#/ with non-string/non-pathname arguments

Ruby 2.2 will define Pathname#/ as a simple alias of Pathname#+.
In practice, this means that it will raise a TypeError unless the
argument responds to to_path or to_str.

Currently we blindly convert the argument to a string using to_s, so
deprecate this in the interest of matching the upstream behavior. In the
future we can replace this with

  alias_method :/, :+ unless method_defined?(:/)

Closes Homebrew/homebrew#30079.

Library/Homebrew/extend/pathname.rb

@@ -301,8 +301,12 @@ class Pathname
     File.symlink(src.relative_path_from(dirname), self)
   end
 
-  def / that
+  def /(other)
-    self + that.to_s
+    unless other.respond_to?(:to_str) || other.respond_to?(:to_path)
+      opoo "Pathname#/ called on #{inspect} with #{other.inspect} as an argument"
+      puts "This behavior is deprecated, please pass either a String or a Pathname"
+    end
+    self + other.to_s
   end unless method_defined?(:/)
 
   def ensure_writable