Update chef-zero api to match the expectation that new user with different case email as an existing user cannot be created.

Before #2400 changes:

If

user1 - USER@abc.com exists
user2 - User@abc.com could be created

then get on api?email=USER@abc.com would return both users.

After #2400

If

user1 - USER@abc.com exists
user2 - User@abc.com cannot be created

hence get on api?email=USER@abc.com would return only the 1 existing user.

If the users already existed then get on api?email=USER@abc.com would return both users <- this makes sure current sraed is not broken.

This API change needs to be reflected into chef-zero to maintain compatibility with server. We currently disable the test with https://github.com/chef/chef-zero/pull/311.

NOTES:
How can I run chef-zero pedant?
on the dev-vm
#cd /opt/opscode/embedded/service/oc-chef-pedant
/opt/opscode/embedded/service/oc-chef-pedant# bundle exec rake chef_zero_spec

How can I test only the failing test:
Add a tag to the test to be focussed on
Include the tag in
/opt/opscode/embedded/service/gem/ruby/2.6.0/gems/chef-zero-15.0.4/spec/run_oc_pedant.rb#L200

NOTE:
This does not break the expectation that if those 2 users existed before the change was applied, get to the users endpoint
Acceptance criteria:
Chef-zero maintains api compatibility with chef-server and returns a 409 conflict for a newuser if (upper or lowercase) email exists

Start with the chef-zero pedant suite using bundle exec rake chef_zero_spec on the dev-vm, then inspect the focused-test configuration at spec/run_oc_pedant.rb#L200. Restore the disabled email-case expectation so a new conflicting user returns 409 while pre-existing users with different casing are both returned.