Support CharSequence under (at least) Scala and RxJava

Currently the CharSequence methods in HttpServerResponse are marked @GenIgnore. This makes it awkward to use the `io.vertx.core.http.HttpHeaders` constants, which are `CharSequences`, with the generated versions (e.g., `io.vertx.reactivex`).

`CharSequence` should be supported by Java, Scala, Groovy, Kotlin, Ceylon, and anything else on the JVM. It seems a shame to limit those, not to mention the various Java APIs, simply because there's no `CharSequence` in JavaScript or Ruby. If anything, it would make more sense to _only_ have the `CharSequence` methods, since a `String` is also a `CharSequence`, and convert them to strings in the non-JVM code generators.

(See discussion in eclipse/vert.x#2211.)