Follow-up to  BUILD-310.  Only Servlet 4.0 which we preferred not to update in order to perform more extensive tests on some possibly breaking changes it introduces, e.g. ServletOutputStream

Some input from Mika regarding the two new methods we need to implement:

• isReady() should return false. I understand it as only leveraged in the context of the non-blocking I/O; and that if you don't use a write listener, then it's not gonna bother checking for readiness; as such I understand [javadoc] as "it returns true if a [non-blocking] write to this ServletOutputStream will succeed"
• then throwing an UnsupportedOperationException in setWriteListener() seems legit to make sure no client usage tries to do that on those stub output streams.