Replace fs.existsSync in afterSend test.
Replace fs.existsSync in afterSend test.

fs.exists and fs.existsSync will be deprecated. Replaces the test case with a
test making sure that the file can be opened for reading.

gustavnikolaj/express-interceptor,papandreou/express-interceptor,axiomzen/express-interceptor,papandreou/express-interceptor