Update document.ts to offer set options, closes #1204

[iclems]

Checklist

• Issue number for this PR: Allow to perform set with additional parameters (e.g. { merge: true }) angular/angularfire#1204 (required)
• Docs included?: no
• Test units included?: no
• In a clean directory, npm install, npm run build, and npm test run successfully? yes

Description

Fixing a missing capability: add options to a .set(data, { merge: true }) call

Code sample

Now, when a developer wants to set data and merge with potential existing value, he can use .set(data, { merge: true }) if he doesn't know if the object already exists.