Force value coercion for `java.util.Properties`, so that values are `String`s

[cowtowncoder]

Currently there is no custom handling for java.util.Properties, and although it is possible to use it (since it really is a Map under the hood), results are only good if values are already Strings.
The problem here is that Properties is actually declared as Map<String,Object>, probably due to backwards-compatibility constraints.

But Jackson should know better: perhaps by TypeFactory tweaking parameterizations a bit?