clone
Description:
Clones the this as a new MessageInfo and eventually assigns a new FolderSummary to it.
If it's not set, then the same summary as the one with this is used.
Parameters:
| this | a MessageInfo to clone | 
| assign_summary | parent FolderSummary object, or null, to set on the clone | 
Returns:
| a new MessageInfo object, clone of the this |