Bug when implementing multiple IReactToDataSystem<T>

[Fijo]

I just noticed a bug when using more than one IReactToDataSystem<T> on a single system.

Sadly to solve it properly it also requires changes to SystemsRx. So I went ahead and just created a unit test to demonstrate it for now.

That test could be shorter but I wanted it to be a somewhat proper one.

[grofit]

Will take a look tomorrow chief, thanks again for raising all this stuff, it may also effect other systems as I never really accounted for multiple of the same interface but its quite possible in some scenarios.

[grofit]

I have added a PR for this fix if you are happy to cast an eye over it before I approve it, the test passes now.

https://github.com/EcsRx/ecsrx/pull/36

[grofit]

This has been merged in now