Work around encoding issue in Pathname#inspect on Ruby 2.0

Pathname#inspect on Ruby 2.0 throws away the encoding of the object's
underlying string and returns a string tagged as ASCII-8BIT.

If you simply write

  puts Pathname.new("some string with non-ascii bytes").inspect

no error will be raised, because the implementation of Pathname#inspect
does not call into Object#inspect.

However, if you wrap that pathname object in an array first, then

  puts [Pathname.new("some string with non-ascii bytes")].inspect

will raise Encoding::CompatibilityError: "inspected result must be ASCII
only or use the same encoding with default external".

Raising an error in this codepath is new in Ruby 2.0, and this specific
bug is fixed in Ruby 2.1. I've opened a bug upstream:
  https://bugs.ruby-lang.org/issues/9915

Fixes Homebrew/homebrew#29947.

Library/Homebrew/extend/pathname.rb

@@ -424,6 +424,13 @@ class Pathname
       end
     end
     private :prepend_prefix
+  elsif RUBY_VERSION == "2.0.0"
+    # https://bugs.ruby-lang.org/issues/9915
+    prepend Module.new {
+      def inspect
+        super.force_encoding(instance_variable_get(:@path).encoding)
+      end
+    }
   end
 end