thrift column definition list sometimes immutable

When the input cf_def.column_metadata is null, ColumnDefinition.fromThrift can return Collections.emptyList() (which is defined to be an immutable list)

One caller, CFMetaData.internalFromThrift , incorrectly assumes that this list will always be mutable and tries to add to this immutable list, which causes the Cassandra server to throw an exception.

It looks like this is a bug in 2.1, but not in trunk.
I can include my one-liner patch, but I'm not sure whether it is better to fix the caller (to not assume random lists you get back from a method are mutable) or the callee (to always return a mutable list).