Fix ws IndexValue parse

[justinpolygon]

When subscribed to the real-time value for a given index ticker symbol with python it was throwing an error saying unknown message type. So, updated the parse_single to recognize the event type and then updated the IndexValue class to return the correct message type.

Here's a test script:

from polygon import WebSocketClient
from polygon.websocket.models import WebSocketMessage, Market
from typing import List

client = WebSocketClient(market=Market.Indices)

client.subscribe("V.I:SPX") # Standard & Poor's 500

def handle_msg(msgs: List[WebSocketMessage]):
    for m in msgs:
        print(m)

client.run(handle_msg)