Assume context is an object

[lorensr]

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