Set Content-Length to 0 when no data is present

The Selenium grid hub will throw an exception if it gets a DELETE or
POST (possibly others) with no Content-Length, even if the request has
no data.

(cherry picked from commit 3ce4c611d015c56c890b05e2dbde925caa293e23)
Add description to recent Content-Length update

(cherry picked from commit 2761948213a3cedc4b3a9315d677ff263169abe6)