Fixed serving of static files under IIS 6 / IIS 7 classic pipeline.

This fixes issue #381. `context.RewritePath` does not work properly for serving static files on IIS 6 (or IIS 7 with classic pipeline). Cassette is setting an ETag, and the framework `StaticFileHandler` also tries to set an ETag, which causes this error. However, even with the ETag line commented out, most of the headers (like the expiry date) were not set correctly on IIS 6.

I've changed the code to continue using `context.RewritePath` when using the integrated pipeline, and fall back to `context.Response.TransmitFile` for IIS 6.