Add a simple unit test for async writes.
This (unexpectedly) passes on all three connectors.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1477644 13f79535-47bb-0310-9956-ffa450edef68
Refactor canWrite -> isReady to align with changes made to Servlet spec

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1477657 13f79535-47bb-0310-9956-ffa450edef68