fixed reference issue in the roslyn script engine

There were two problems that I've noticed.

1. The UnionWith from the script pack references at line 49 would affect the AssemblyReferences object passed. This seems like an undesirable side effect. At least this is not something I want in my ScriptCs hosting.

2. The way references were added if the session already exists had a bug. Every other time, if there were no new refernces, all passed refs were added again because the ```session.References``` was then empty. This was the culprit: ```sessionState.References = newReferences;```. This created quite a bit of a lag in my REPL.