Prevent OPTIONS route clobber

Moved the IsOptionsRequest conditional branch to only execute if no
defined OPTIONS route for the context exists. Otherwise, even if you
define an OPTIONS route manually, the auto-OPTIONS conditional clobbers
the Response, preventing custom additions such as CORS headers.

Addresses discussion at https://github.com/NancyFx/Nancy/commit/604327a00286f39f925840c2b1678c69e2d67626#commitcomment-3148154