test/rgw: pass encoding to Key.get_content_as_string()

test/rgw: pass encoding to Key.get_content_as_string()

we assume that boto.Key.get_content_as_string() returns str instead of
bytes, and compare the return value with a string, so, to ensure that
lhs and rhs are both strings, we need to decode the returned content.

since we always store strings composed with ASCII, it's safe to use
'ascii' to decode them.

Signed-off-by: Kefu Chai <kchai@redhat.com>