SPR-7591 introduced a java.nio.charset.Charset field within
HttpStatusCodeException. The former is non-serializable, thus by
extension the latter also became non-serializable.
Because the Charset field is only used for outputting the charset name
in HttpStatusCodeException#getResponseBodyAsString, it is reasonable to
store the value returned by Charset#name() instead of the actual Charset
object itself.
This commit refactors HttpStatusCodeException's responseCharset field to
be of type String instead of Charset and adds tests to prove that
HttpStatusCodeException objects are once again serializable as expected.
Issue: SPR-9273, SPR-7591
2ff43726be