use builtin header for scoped v4 clients

[wildemat]

Ticket(s): FE-5698

Problem

• scoped v4 clients weren't getting the "builtin" header

Solution

• use the getHeaders() function from the other places we build a v4 client

Result

• all methods of building a v4 client employ the "x-fauna-shell-builtin" header check

➜  fauna-shell git:(createkey-fix) ✗ node bin/run create-key made_in_v4
Connected to endpoint: devlocal-us
Connected to endpoint: devlocal-us database: made_in_v4
creating key for database 'made_in_v4' with role 'admin'

  created key for database 'made_in_v4' with role 'admin'.
  secret: <redacted>

  To access 'made_in_v4' with this key, create a client using
  the driver library for your language of choice using
  the above secret.

Testing

• create-key is one command that uses scoped clients. Updated the mockCreateKey method there to expect the builtin header.