Use Charset instead of encoding string where possible

[mperktold]

Target Jetty version(s)
Jetty 10.0.x

Enhancement Description
We've noticed a few places where a string charset is passed to a JDK method even if it would be possible to directly use a Charset instead:

• HttpRequest#urlEncode
• HttpRequest#urlDecode
• FormRequestContent#encode

Maybe there are some more, but those are the ones we found.

This is more of an observation rather than a feature request.

Also, the corresponding overloads accepting a Charset are only available since Java 10, which is fine for Jetty 10 but not for Jetty 9, so please decide for yourself whether this is worthwhile.

[joakime]

Opened as PR #7137

[joakime]

Merged into jetty-10.0.x and jetty-11.0.x