Handle OAuth2Error when inherited from base Error

Related to https://github.com/thomseddon/node-oauth2-server/pull/68.
Update node-oauth2-server dependency

Version 2.2.1 includes the changes that makes OAuth2Error inherit from
base error.
Update error handler ctx body key exposure
Merge pull request #15 from seegno/feature/handle-oauth2error-inherited-error

Handle OAuth2Error when inherited from base Error
Expose WWW-Authenticate headers
Merge pull request #14 from seegno/feature/expose-www-authenticate-headers

Expose WWW-Authenticate headers