OpenMBeanOperationInfo should have a readResolve method like the one
OpenMBeanAttributeInfo and OpenMBeanParameterInfo have to add the
"ReturnOpenType" into the Descriptor at OpenMBeanOperationInfo´s
deserialization time. This allows OpenMBeanOperationInfo´s coming
from Tiger (without Descriptor) to be reconstructed with a Descriptor.

An object serialized in a version of the API before Descriptors were added
to this class will have an empty or null Descriptor. For consistency with our
behavior in Mustang, we must replace the object with one where the Descriptors
reflect the same values of "ReturnOpenType". The field name to be used in
the Descriptor for the return open type is "openType" as specified by the
javax.management.Descriptor javadoc.