Fix the Javadoc, that was copied and pasted from another Option by mistake.

Verify if there is a value used for this option by default (i.e., if it is not null by default when read). If that's the case, creat a public static final field called UndertowOptions.DEFAULT_URL_CHARSET, and make sure it is used whenever needed.

Add a Javadoc to the new constant in case you created it and make sure it is linked to URL_CHARSET with @link