Due to changes made in commit 2fa87a71 for SPR-9118,
AbstractResource#contentLength would fall into an infinite loop unless
the method were overridden by a subclass (which it is in the majority of
use cases).
This commit:
- fixes the infinite recursion by refactoring to a while loop
- asserts that the value returned from #getInputStream is not null in
order to avoid NullPointerException
- tests both of the above
- adds Javadoc to the Resource interface to clearly document that the
contract for any implementation is that #getInputStream must not
return null
Issue: SPR-9163
Backport-Issue: SPR-9161
Backport-Commit: 7ca5fba05f
40a6769309