Update VcapApplicationListener to support Number based credentials.
Previously, any service credential value that wasn't a String would
be discarded. This was particularly a problem for services that exposed
a port as a JSON Number.  This change takes numbers in the credential
payload into account, converting them to Strings so that they will pass
through the properties system properly. There's no real downside to this
as Spring will coerce them back into Numbers if needed by an application.

Fixes gh-2707