[fix] mirrors heads are not rebounds

rfc/0004_ws2p_v1.md

@@ -308,6 +308,7 @@ When the local node receives a HEAD message, it checks whether all of the follow
 2. The head has a block number equal to or greater than all cached heads for the same ws2pid  
 3. If the block number is equal to the cached head, the head is of a higher or equal version.  
 4. If the block number is equal to the cached head, and the version is identical, the step field is smaller.
+5. If the head pubkey is not a member, make sure there is an active ws2p connection with this pubkey.
 
 If all these conditions are respected, perform the following actions :  
 1. Save this head in the headCache, overwrite the existing head if it exists.