The raw directive currently defers the decision of whether it can handle
a container to its subject (handles_container). If the subject cannot
handle a container, the raw directive returns false too. Since
is_container returns true for iterator_range, this creates problems with
the sequence operator that tries to call
attr.insert(attr.end(), raw_range) when handles_container of the raw
subject is false. Since the raw directive can handle container
attributes (it has for some time), it seemed best to always return true
for handles_container.
