Fixes incorrect use of Object.assign when backfilling SSRC config with defaults.

[erikeldridge]

In the logic where we backfill config with defaults, the first argument to Object.assign should be an object to assign to, but the code passed the object containing the defaults. It should be:

const mergedConfig = Object.assign({}, this.defaultConfig, evaluatedConfig);

Not:

const mergedConfig = Object.assign(this.defaultConfig, evaluatedConfig);

The effect of the bug was to overwrite the defaults with the remote values, which is why in-app default would initially work, and updating to a remote value would work, but then switching back to in-app default wouldn't change the value from the previous remote value.

Discussion

Working with @lahirumaramba and @amanda-xia.

Testing

Ran npm test and all tests pass.

Functionally tested using a local server.