[mining] segwit option must be set in GBT

Calling getblocktemplate without the segwit rule specified is most
likely a client error, since it results in lower fees for the miner.
Prevent this client error by failing getblocktemplate if called without
the segwit rule specified.

test/functional/mining_prioritisetransaction.py

@@ -142,10 +142,10 @@ class PrioritiseTransactionTest(BitcoinTestFramework):
         # getblocktemplate to (eventually) return a new block.
         mock_time = int(time.time())
         self.nodes[0].setmocktime(mock_time)
-        template = self.nodes[0].getblocktemplate()
+        template = self.nodes[0].getblocktemplate({'rules': ['segwit']})
         self.nodes[0].prioritisetransaction(txid=tx_id, fee_delta=-int(self.relayfee*COIN))
         self.nodes[0].setmocktime(mock_time+10)
-        new_template = self.nodes[0].getblocktemplate()
+        new_template = self.nodes[0].getblocktemplate({'rules': ['segwit']})
 
         assert(template != new_template)