rpc, test: document {previous,next}blockhash as optional

Affects the following RPCs:
- getblockheader
- getblock

Also adds trivial tests on genesis block (should not contain
"previousblockhash") and best block (should not contain
"nextblockhash").

test/functional/rpc_blockchain.py

@@ -304,6 +304,9 @@ class BlockchainTest(BitcoinTestFramework):
         header.calc_sha256()
         assert_equal(header.hash, besthash)
 
+        assert 'previousblockhash' not in node.getblockheader(node.getblockhash(0))
+        assert 'nextblockhash' not in node.getblockheader(node.getbestblockhash())
+
     def _test_getdifficulty(self):
         difficulty = self.nodes[0].getdifficulty()
         # 1 hash in 2 should be valid, so difficulty should be 1/2**31
@@ -408,6 +411,9 @@ class BlockchainTest(BitcoinTestFramework):
         # Restore chain state
         move_block_file('rev_wrong', 'rev00000.dat')
 
+        assert 'previousblockhash' not in node.getblock(node.getblockhash(0))
+        assert 'nextblockhash' not in node.getblock(node.getbestblockhash())
+
 
 if __name__ == '__main__':
     BlockchainTest().main()