Consider Pathname object as String, rather than stream when MiniMagick::Image#write

When `MiniMagick::Image#write` to a `Pathname` object such as `Rails.root.join('image/sample.jpg')`,  an `Encoding::UndefinedConversionError ("\x89" from ASCII-8BIT to UTF-8)` error occurs. The mini_magick takes a Pathname object as stream, which also respond_to :write but not act as expected. And passing a Pathname object to `MiniMagick::Image#write` method is reasonable.