Restore the previous `spring.profiles.active` logic effectively
reverting commit 37069d9. This ensures that active profiles defined in
a higher `PropertySource` replace those lower down.
In addition, add support for a `+` prefix that can be used to indicate
that a profile should be added to the active set.
For example:
# application-prod.properties
spring.profiles.active=+proddb,+prodmq
Fixed gh-308, gh-309