test case for serializing custom numbers with config overrides

[pjfanning]

• relates to a broken test in jackson-module-scala - Create NumberSerializerTest.scala jackson-module-scala#748 - that 748 test basically works with Jackson 2 (and is merged in jackson-module-scala 2.x branch)
• Scala has scala.math.BigDecimal which is a wrapper on java.math.BigDecimal but we don't want test cases in jackson-databind that bring in outside dependencies. The MyBigDecimal class in this PR is enough to trigger the issue
• of the 2 unit tests, the BigDecimal one already works but the MyBigDecimal one fails
• I did a little debugging and the code in BasicSerializerFactory.java to check the config overrides has changed quite a bit in Jackson 3 and the changes cause this test case to fail
  • I think the issue is around here: https://github.com/FasterXML/jackson-databind/blob/3.x/src/main/java/tools/jackson/databind/ser/BasicSerializerFactory.java#L314-L316
  • the formatOverrides passed in is null and the hardcoded Number.class in the _check call leads to the formatOverrides for MyBigDecimal.class not being found
  • there are a few changes that I have tried that fix the test. I have included 1 in this PR for illustration

[cowtowncoder]

This seems like patching a bug further upstream? I'll have to think about this bit more -- after vacation (going on vacation today), so it'll be some time until I get back, but will go through it then.

[cowtowncoder]

Would probably work without this part too, but seems more correct to specify handled type as real type, not Number.

[cowtowncoder]

@pjfanning I changed the solution slightly, will now merge. While it seems like override target ideally should be BigDecimal, this is the way 2.x works & NumberSerializer would otherwise need to sort of enumerate "proper" handled types available (to distinguish JDK types like BigDecimal, BigInteger from custom subtypes) and... things would get bit complicated.