Handle missing environment from non empty config

If using a `DATABASE_URL` and a `database.yml`. The connection information in `DATABASE_URL` should be merged into whatever environment we are in. As released in 4.1.0rc1 if someone has a database.yml but is missing a key like production:

```yml
development:
  host: localhost

```

Then the check for blank config will return false so the information from the `DATABASE_URL` will not be used when attempting to connect to the `production` database and the connection will incorrectly fail.

This commit fixes this problem and adds a test for the behavior.

In addition the ability to specify a connection url in a `database.yml` like this:

```
production: postgres://localhost/foo
```

Was introduced in 4.1.0rc1 though should not be used, instead using a url sub key

```
production:
  url: postgres://localhost/foo
```

This url sub key was also introduced in 4.1.0rc1 though the `production: postgres://localhost/foo` was not removed. As a result we should not test this behavior.
Merge pull request #14236 from hone/4-1-stable

Backport 283a2ed from master to 4-1-stable