Enhance put_object to inform when chunk is ignored

Changed documentation for chunk_size parameter to indicate
that it can be used only with file like objects.

Also added a UserWarning when using a string contents to inform
that chunk_size will be ignored.

Added a unit test to check whether the warning is working
correctly.

Fixes: bug #1147232
Change-Id: I618ec45520ba81905ce2ead4d61f192d21ae3489
Merge "Enhance put_object to inform when chunk is ignored"