ApolloCreek

classes Message and SystemPropertiesCollection must provide abstractions to allow testing

2

issue: [Message](https://github.com/Azure/azure-service-bus-dotnet/blob/df3156c0a0987c87d0da9ef5c07d33694070c034/src/Microsoft.Azure.ServiceBus/Message.cs#L17) has no abstractions (either interface or abstract base class), but it has 'internal' setter and uses the sealed (why the heck is this?!) [SystemPropertiesCollection](https://github.com/Azure/azure-service-bus-dotnet/blob/df3156c0a0987c87d0da9ef5c07d33694070c034/src/Microsoft.Azure.ServiceBus/Message.cs#L344) class. This _completely_ avoids...