Always pass an encoding option to Zlib::GzipReader.wrap

The arity of this method has been -1 since the import, so the
option has been passed always, even if Zlib::GzipReader#initialize
does not take the option.  Actually it takes the option since 1.9.

lib/rubygems/package.rb

@@ -534,11 +534,7 @@ def load_spec(entry) # :nodoc:
     when 'metadata' then
       @spec = Gem::Specification.from_yaml entry.read
     when 'metadata.gz' then
-      args = [entry]
-      args << { :external_encoding => Encoding::UTF_8 } if
-        Zlib::GzipReader.method(:wrap).arity != 1
-
-      Zlib::GzipReader.wrap(*args) do |gzio|
+      Zlib::GzipReader.wrap(entry, external_encoding: Encoding::UTF_8) do |gzio|
         @spec = Gem::Specification.from_yaml gzio.read
       end
     end