Allow serialization of `Properties` with non-String values

[cowtowncoder]

(for background, see #2656 and #810)

Currently, code like so:

        Properties properties = new Properties();
        properties.put("key", 1);
        String json = new ObjectMapper().writeValueAsString(properties);

will fail with an exception because all values of Properties are assumed to be of type String.
This is due to typing fix made for #810 which leads to selection of StringSerializer -- reasonable for most cases but not all.

While use of Properties for non-String values seems like a wrong thing to do, maybe we could still support it similar to how coercion of non-String values from JSON content works with Properties (for scalars, at least).

[fan2008shuai]

Hmmh, Thank you for your reply.
That's tricky. Some old code use Properties like a hash map, put non-string into it, unfortunately Properties#getProperty return String. It is conflictive. Maybe following JDK is a good idea.

[cowtowncoder]

Improved code for 2.11(.0) so that serialization actually succeeds.