Refactoring & minor changes

Reviewed the code for Pipeline and made a few small changes (cleanup), added 2 tests for edge case scenarios, and simplified logic for [PipelineBuilder] (https://github.com/seanfeldman/NServiceBus/blob/30ba741eab63c0d5f8005e691c2f3aa448ec1107/src/NServiceBus.Core/Pipeline/PipelineBuilder.cs#L28) Incoming & Outgoing Behaviors