`BaseStreamWriter.writeRaw(String)` should not close open element (add overload of `writeRaw()`)

This is copy of #95 whose fix (introduced in 7.2.0) was reverted in 7.2.1. There should be some method which allows to emit whitespace within the currently open tags. As both `writeRaw(...)` and `writeSpace(...)` (calls the former) close the currently open tag those cannot be used for that purpose.
One has to be careful to not cause regressions, so it is probably best to introduce either a new method or clarify the javadoc which method closes the tag and which one doesn't.