JenkinsHttpClient post_form_with_result character set

### What feature do you want to see added?

Hello，I‘m using this the api can't change the charset，hope to support changes to the utf-8 character set。

### Upstream changes

_No response_

Start by locating JenkinsHttpClient.post_form_with_result and inspect how its character set is currently selected. Done means the API can support changing the character set to UTF-8, with the resulting request behavior verified where the client’s existing tests cover this method.

索引モデルが issue の本文から書いたものです。