Set default http status of a successful ServiceResponse

What does this MR do?

Sometimes you want to just pass the http_status of the service response despite its success/failure status.

Before

  result = service.execute

  if result.success?
    head :ok 
  else
    head result.http_status
  end

After

  result = service.execute

  head result.http_status

Came up during a review https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/16508#note_216281609

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team