fix user overrides: merge overrides with current user attributes

[Sillhouette]

This pull request addresses the issue that is laid out by my colleague @dtothefp here: optimizely#120

It will merge the existing user attributes with the current user's attributes to prevent losing the user attributes that are set when the optimizely client is initialized.

[mikechu-optimizely]

Thanks for relating this to @dtothefp 's Issue and putting in the coding for it.

I've related and upgraded the internal ticket (FSSDK-8652).

When you have a chance, please review the CONTRIBUTING.md and sign the linked Contributors License Agreement (CLA).

[Sillhouette]

Thanks for relating this to @dtothefp 's Issue and putting in the coding for it.

I've related and upgraded the internal ticket (FSSDK-8652).

When you have a chance, please review the CONTRIBUTING.md and sign the linked Contributors License Agreement (CLA).

I've done that 👍

[junaed-optimizely]

I have already given my thought on this here in the issue comment. I think the way we handle overrides are intended this way.

But its an interesting idea, and we might revisit it in future if required.

Thanks!