Fixing Case Handling for Volume Resource

[T-jegou]

Hello, this Pull Request is intended to address the issue #901. However, I've observed that the API V2 accepts, for certain types of resources, the region to be provided in uppercase.

In my Pull Request, I've implemented a solution for the volume resource, involving converting the region string to lowercase when it's added to the opts structure in the resourceDigitalOceanVolumeCreate function, to ensure that the provider calls godo with the region field in lowercase.

However, I find that this approach lacks consistency since the API handles the case of a region field in uppercase in the context of the droplet resource (and probably others).

Perhaps the fix should be applied directly to the API.

Nevertheless, strictly adhering to the API documentation, the region slugs are only in lowercase. In the case where a region is indicated in uppercase, either in the resource schema, we apply a condition to ensure the region is only in lowercase, and the documentation should state "lowercase only" for region slugs, or we accept and handle the formatting to lowercase for storage in the state (with StateFunc) and for sending data to godo in opts.

[andrewsomething]

Hey @T-jegou! Thanks for the PR. I agree that it would be better if the API handled this consistently, and we can share that feedback internally. Though I think this change is helpful for Terraform users in the meantime.