Test coverage around JsonMessageSerializer and JsonConverterAttributes

As [discussed on Google Groups](https://groups.google.com/forum/?fromgroups#!topic/masstransit-discuss/9YIFBeCewxI), it seems that because of the way `JsonMessageTypeConverter` creates the Message object using `FastActivator.Create`, any `JsonConverterAttribute` the message might be decorated with is not honoured *when deserializing*.

These test try to show that by attempting to modify simple messages upon serialization and (separately) deserialization. 

Better story with properties, their `JsonConverterAttribute`s are honoured both when serializing, as well as deserializing.

(My first pull request so let me know if I do something the wrong way!)