Skip convertToR search for values with are SymbolicExpressions

[glchapman]

Currently, RInteropInternal.convertToR searches through its various conversion methods even when passed a value which is already a SymbolicExpression (or a subclass of SymbolicExpression). It seems like it might be a useful optimization to check for SymbolicExpressions first, and skip the search if one is found.

[tpetricek]

This sounds like a good idea! If you can submit a pull request, that would be great!

PS: Sorry for a slow ping, I'm finishing a new release and finally going through all the old issues....