Do not encode single/double quotes as entities

By encoding single/double quotes we can potentially break input, so lets
stop doing this. This now ensures that this:

    <foo>a"b</foo>

Is actually serialized back into the exact same instead of being
serialized into:

    <foo>a&quot;b</foo>

lib/oga/xml/entities.rb

@@ -28,8 +28,6 @@ module Oga
       #
       ENCODE_MAPPING = {
         '&' => '&',
-        '"' => '"',
-        "'" => ''',
         '>' => '>',
         '<' => '&lt;',
       }