Allow override of raise_errors

Small change to let you override the `:raise_errors` option for an individual request. I added a spec for this to demonstrate the desired behavior as well. There are many APIs that return 400 error codes that are not intended to be exceptional. My use case is the GitHub API, which uses 204/404 in places like checking for [team membership](http://developer.github.com/v3/orgs/teams/#get-team-member).