Fix database configurations building when DATABASE_URL present

[palkan]

Summary

Rails Version: 6.0.0.beta1

Having the following database.yml:

default: &default
  adapter: postgresql
  encoding: unicode
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>

development:
  <<: *default
  database: project_development

test:
  <<: *default
  database: project_test

production:
  <<: *default
  database: project_production

I got the following exceptions running tests (after calling maintain_test_schema!):

PG::ConnectionBad:
#   FATAL:  database "project_development" does not exist
#   /bundle/gems/pg-1.1.4/lib/pg.rb:56:in `initialize'

After some investigation I found that DB configurations all had "test" as env_name.
And specifically for test env:

[6] pry(main)> resolver = ActiveRecord::ConnectionAdapters::ConnectionSpecification::Resolver.new(ActiveRecord::Base.configurations)
=> #<ActiveRecord::ConnectionAdapters::ConnectionSpecification::Resolver:0x000055b27e697590
 @configurations=
  #<ActiveRecord::DatabaseConfigurations:0x000055b27e5febb0
   @configurations=
    [#<ActiveRecord::DatabaseConfigurations::UrlConfig:0x000055b27bec2830
      @config=
       {"adapter"=>"postgresql",
        "encoding"=>"unicode",
        "pool"=>5,
        "database"=>"project_development",
        "username"=>"postgres",
        "password"=>"postgres",
[7] pry(main)> resolver.resolve(:test, :test)
=> {"adapter"=>"postgresql",
 "encoding"=>"unicode",
 "pool"=>5,
 "database"=>"project_development",
 "username"=>"postgres",
 "password"=>"postgres",
 "port"=>5432,
 "host"=>"postgres",
 "name"=>"test"}

This is a regression comparing to 5.2.

NOTE: For those struggling with this problem: it could be fixed by adding url

default: &default
  url: <%= ENV['DATABASE_URL'] %>