[feature] Node#write_to and friends should accept Encoding objects for :encoding kwargs

[flavorjones]

Currently these methods accept an :encoding arg, whose value is the name of the encoding:

node.write_to(io, encoding: "UTF-8")

When passing in an Encoding object, though, we get an error:

node.write_to(io, encoding: Encoding::UTF_8)
# TypeError: no implicit conversion of Encoding into String
#    .../lib/nokogiri/xml/node.rb:1368:in `native_write_to'
#    .../lib/nokogiri/xml/node.rb:1368:in `write_to'

We should handle Encoding objects wherever we accept encoding name strings.