Test coverage

[helfer]

We should make sure all significant logic is covered by tests. Right now the line coverage is close to 100%, but it seems some features aren't tested.

[helfer]

Options currently not covered by tests:

• logFunction
• formatError
• validationRules
• rootValue

logFunction can't be tested yet because it doesn't actually do anything as long as logging PR isn't merged in graphql-js (graphql/graphql-js#402)

[helfer]

In addition, the following should be done:

• remove unused resolve functions in tests for OperationStore.
• test the getMap function in OperationStore
• test option sanity checks in expressApollo
• figure out why status code 400 response shows up as not covered in HAPI integration
• figure out why status codes 400 and 405 show up as not covered in Koa integration