Fix regression in generate_etag function in riak_cs_wm_object

This regression snuck in as part of
e827ff1ce415b51d4aa374c16c3c00fa381833a1.  The etag returned from
generate_etag should not be quoted. This caused the
object_get_conditional_test riak_test module to fail.
Merge branch 'bugfix/quoted-etag-regression' into develop