Fix: #2057 Saga handler called twice with MultipleHandlerBehavior.Separated

[scrodde]

Fixes #2057

In SagaChain.cs, the constructor for sticky endpoints was adding the handler to the Handlers list twice:

1. Implicitly via base(handlerCall, handlerGraph) which calls Handlers.Add(call) in the base HandlerChain constructor
2. Explicitly on line 64 with Handlers.Add(handlerCall)

This caused the handler to appear twice in code generation, resulting in the saga's Handle() method being called twice in the generated code.

[jeremydmiller]

Doh. Good catch.