[RPC] Fix casing in getblockchaininfo to be inline with the rest of the response

The response in the RPC result `starttime` is camel cased while the rest of the response seems to be lower cased.

If this was intentional please ignore this PR.

Note: case might break existing callers

Reflect the change in the test data

Change to snake case

test/functional/rpc_blockchain.py

@@ -134,7 +134,7 @@ class BlockchainTest(BitcoinTestFramework):
                 'bip9': {
                     'status': 'started',
                     'bit': 28,
-                    'startTime': 0,
+                    'start_time': 0,
                     'timeout': 0x7fffffffffffffff,  # testdummy does not have a timeout so is set to the max int64 value
                     'since': 144,
                     'statistics': {