Allow a single serializer class per JSONAPISource.
Allow a single serializer class per JSONAPISource.

The serializer must understand how to serialize multiple types in order
to work with compound documents. Therefore, it doesn’t make sense to
have a `serializerForType` on the `Source`.
SmuliS/orbit.js,lytbulb/orbit-firebase,ProlificLab/orbit.js,opsb/orbit-firebase,SmuliS/orbit.js,greyhwndz/orbit.js,opsb/orbit-firebase,jpvanhal/orbit-core,ProlificLab/orbit.js,gnarf/orbit.js,lytbulb/orbit.js,orbitjs/orbit.js,lytbulb/orbit-firebase,jpvanhal/orbit.js,opsb/orbit.js,opsb/orbit.js,beni55/orbit.js,opsb/orbit-firebase,rollokb/orbit.js,jpvanhal/orbit.js,rollokb/orbit.js,beni55/orbit.js,opsb/orbit-firebase,greyhwndz/orbit.js,orbitjs/orbit.js,orbitjs/orbit-core,lytbulb/orbit-firebase,lytbulb/orbit.js,jpvanhal/orbit-core