Previously, IsWriteableArrayHandle just checked to see if an
ArrayHandle's portal has a ValueType of void* because we had coded the
special read-only array handles to have fake portals for writing.
However, we recently removed that because it was more trouble than it
was worth. Now IsWritableArrayHandle checks for the existance of the Set
method. If it does not exist, then the portal is considered read-only.

Also corrected spelling (writeable -> writable).