Update `DataSourceHealthContributorAutoConfiguration` so that any
`AbstractRoutingDataSource` beans are still included in the overall
health. Prior to this commit, a regression in Spring Boot 2.2 meant
that if a single routing bean was found an `IllegalArgumentException`
would be thrown.

In Spring Boot 2.1 all `AbstractRoutingDataSource` would be filtered
from the results, but if no results existed the following was returned:

  "details": {
    "db": {
      "status": "UNKNOWN"
    },

In Spring Boot 2.2 we now always include routing datasource beans, even
if other non-routing database beans are found. The health details
includes `"routing" : true` to help users disambiguate any results.

Fixes gh-18661