Use the (undocumented) URI.get_encoding method introduced in Ruby 2.1 to
look up encodings by the aliases specified in HTML5. This means that the
behavior will differ slightly between versions of Ruby, but the
encodings selected are largely compatible.
For example, `ISO-8859-1` is an alias for `Windows-1252` per the HTML5
specification, while in ruby versions < 2.1 it will be used as is. These
two encodings are largely compatible, and the alias exists due to
servers that return a `charset=ISO-8859-1` when they actually are using
`Windows-1252`.
Other aliases that differ include `shift_jis` (rendered as
`Windows-31J`) and `euc-jp` (rendered as `CP51932`).
