Rails5 fix db check: connection execute returns integer instead of string

What does this MR do?

In Rails 5 the check returns 1 instead of '1'. A simple &.to_s fixes the problem.

Fixes these specs:

rspec ./spec/controllers/health_controller_spec.rb[1:2:1:1:1]
rspec ./spec/controllers/health_controller_spec.rb[1:2:2:2:1:1:1]
rspec ./spec/controllers/health_controller_spec.rb[1:2:2:3:1:1]
rspec ./spec/lib/gitlab/health_checks/db_check_spec.rb[1:3:1]

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

rails5 upgrade

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?