README.md: Add missing comma

Updating TypeScript definitions

Fixing doc on accessing response with then/catch

Fixing xsrf header on missing xsrfCookieName

Docs were suggesting that accessing the response with `catch` was the same as accessing it with `then`. Split the two cases and point to the Handling Errors section. Also mention the case where a rejection callback is specified as second parameter of `then`.

Fixing issue with auth config option and Authorization header

The http adapater did not remove a custom Authorization header when auth is set.

Replace 'agent' option with 'httpAgent' and 'httpsAgent'

closes #378

Changing adapter signature to receive config and return promises

Enhance adapter errors