fix(middleware-location-constraint): insert LocationConstraint only additively

[kuhe]

Issue

n/a

Description

for S3 CreateBucket, we have functionality to automatically set CreateBucketConfiguration.LocationConstraint.
This PR makes this functionality purely additive, rather than potentially overwriting an existing object.

Testing

new integration test

Checklist

• If the PR is a feature, add integration tests (*.integ.spec.ts).
• If you wrote E2E tests, are they resilient to concurrent I/O?
• If adding new public functions, did you add the @public tag and enable doc generation on the package?