fix(database): allow null values for equalTo, etc. - #715

Checklist

• Issue number for this PR: Queries with null don't work as expected #704
• Docs included?: no
• Test units included?: no
• e2e tests included?: no
• In a clean directory, npm install, npm run build, and npm test run successfully? no

After following the steps in the contribution guide, the generated test-root.umd.js bundle was empty - apart from the boilerplate. This could have something to do with Windows being my OS. Hopefully, Travis will run the tests for me.

Description

This PR includes a refactor to rename a method that was considered to be misleading and a fix to allow the equalTo, startAt and endAt options for list queries to be null - as the current AngularFire2 behaviour differs from that of the underlying SDK (see the repro in the associated issue).

I've renamed the method to isNil - rather than the suggested isEmpty - as isNil is the name that's used in lodash (for an identical method) and using isEmpty would make the implementation of isEmptyObject too confusing:

export function isEmptyObject(obj: Object): boolean {
  // If the variable is considered empty, the object isn't? Huh?
  // What does empty mean?
  if (isEmpty(obj)) { return false; }
  return Object.keys(obj).length === 0 && JSON.stringify(obj) === JSON.stringify({});
}