Prefix ragun properties with Serilog$ to decrease likelyhood of key clas...

Hi,

I encountered an issue with duplicate keys when trying to destructure a template parameter called {@Message}. This is due to another parameter named message being added to the parameters sent to Raygun in the Raygun Sink. I've included a change to prefix the additional Message and MessageTemplate parameters with Serilog$. Although this does not completely solve the problem, it is a small/simple change that I feel will make key clashes less likely. A more permanent solution can be devised to be smarter about the Message and MessageTemplate property key generation, but a prefix is an OK stop gap I think.