Add user/userContext to list of properties merged into _globalContext via RavenConfig

[simenbrekken]

Do you want to request a feature or report a bug?

Feature

What is the current behavior?

Raven only handles tags and extra when reading RavenConfig. This makes it impossible to run Raven.setUser when raven is being loaded and configured asynchronously via RavenConfig.

What is the desired behavior?

Include user/userContext in the list of properties that are merged into the _globalContext

This will allow integrations where RavenConfig is used to per-configure raven-js before Raven is loaded to include information about the current user.

[benvinegar]

I encountered this issue personally, and it was pretty confusing. Going to add this for 3.9.0.