Monitoring & Unmonitoring an endpoint without the hearbeat plugin

When monitoring/unmonitoring an endpoint that had no heartbeats, a no heartbeat found exception was thrown. This fix ensures that endpoints that don't have the plugins can also be un-monitored.

**Question**
Any reason, why 
https://github.com/Particular/ServiceControl/blob/develop/src/ServiceControl/CompositeViews/Endpoints/GetEndpoints.cs#L83 
this was always set to true?
ServicePulse is looking at the MonitorHeartbeat value to filter out if an endpoint needs to be displayed or not in the active/inactive list.
@andreasohlund - please review.