add filter for cleaning invalid Accept-Language values

Browsers that have Latin American Spanish selected as a preferred language send `Accept-Language` headers containing `es-419`, which isn't supported in Jersey 1.x (which Dropwizard uses). This PR adds a filter that replaces `es-419` with `es-ES`, which should be good enough. Fixes #308.

/cc @wsorenson @HiJon89 @natea