Assume context is an object

Some methods check `this.context` and some assume it's an object:

https://github.com/apollographql/fullstack-tutorial/blob/master/final/server/src/datasources/user.js

I think it's safe to assume it's an object:

https://github.com/apollographql/apollo-server/blob/875944ea8358caa2140e6f7fff861f7a8f0149e4/packages/apollo-server-core/src/ApolloServer.ts#L587

Start with final/server/src/datasources/user.js and compare its context handling with the ApolloServer.ts reference linked in the issue. Review the methods that check context against those that assume an object, then make their behavior consistent with the stated assumption and verify the affected tutorial flow.

Written by the indexing model from the issue text.