Return sanitized value from req.sanitize()

In older versions of express-validator, the sanitized value was returned
from req.sanitize. With the shift to validator.js v3, that behavior was
lost.

For consistency, req.sanitize() should return the sanitized value, just
like validator.js.
Merge pull request #84 from dpolivy/returnfromsanitize

Return sanitized value from req.sanitize()