This allows for the ability to opt-in to a method to force the
encoding of response bodies.  By setting the accessor to a String
or Encoding instance, it will force the specified encoding.
Setting the value of true will try to detect the encoding of the
response body, either using the Content-Type header (assuming it
specifies charset) or by scanning for a <meta> tag in the document
that specifies the encoding.  The default is false in which case
no forcing of encoding will be done (same as before the patch).

Implements [Feature #2567]
Implements [Feature #15517]

Co-authored-by: Yui Naruse <naruse@ruby-lang.org>