SignalR sends incomplete JSON in socket frames when using new System.Text.Json Serializer

_From @suedama1756 on Tuesday, September 3, 2019 5:02:57 PM_

3.0.100-preview8-013656

SignalR JS Client: 3.0.0-preview6.19307.2

We have an issue where SignalR is sending incomplete JSON in a socket frame to the browser which results in an error in the signalr client.

```js
if (input[input.length - 1] !== TextMessageFormat.RecordSeparator) {
throw new Error("Message is incomplete.");
}
```

The issue does not occur when using the NewtonSoft JSON serializer. It looks like the JSON buffer is not being flushed correctly

_Copied from original issue: dotnet/corefx#40770_