Fix  HttpUrlConnection DELETE without body

The following commit allowed HTTP DELETE with body:
https://github.com/spring-projects/spring-framework/commit/584b831bb9390a49c791d180d023e3a585203215

However it broke buffered requests even without a body since JDK 1.6
and 1.7 do not support calls to getOutputStream with HTTP DELETE.

This commit set the doOutput flag back to false if the actual buffered
body is 0 length.

Issue: SPR-12361