fix: Response.charset does not support RFC 7231 compliant Content-Type headers using quotation marks as application/json; charset="utf-8"

[stefanvitz]

Hi,

when migrating from rest template to feign i noticed that the feign Response object does not determine the charset in an RFC compliant way. As this charset() method is used e.g. by the JacksonDecoder class decoding an response with an RFC compliant Content-Type header like application/json; charset="utf-8" will fail.

See also: https://datatracker.ietf.org/doc/html/rfc7231#section-3.1.1.1

For example, the following examples are all equivalent, but the first is preferred for consistency:

 text/html;charset=utf-8
 text/html;charset=UTF-8
 Text/HTML;Charset="utf-8"
 text/html; charset="utf-8"

[stefanvitz]

thx for the fast processing :)