[JAVA] Java driver throws exception for asNumber for BsonDecimal128 (#500)

* what's new: bsondecimal128 responses match java

Author: rachel-mack

source/whats-new.txt

@@ -54,6 +54,11 @@ The 5.0 driver release introduces the following features:
   name, or it defaults to ``MULTIPLE`` if you specify more than one
   host.
 
+- ``BsonDecimal128`` values respond to method calls in the same way as Java
+  ``Decimal128`` values. In particular, the error responses for the
+  ``isNumber()`` and ``asNumber()`` methods match the Java responses for
+  equivalent ``Decimal128`` values.
+
 .. _version-4.11:
 
 What's New in 4.11