CBG-3412: [3.0.9] Fix panic for assigning to nil map inside Mutable1xBody

[gregns1]

CBG-3412

Backport of the fix for panic found in Mutable1xBody. Had to move the test out of rest package into the db package as function parameters have changed since 3.1 and made more sense for it to be here as rest tester was no longer needed.

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/2013/