MONGOID-5507 - Fix two bugs in BigDecimal mongoization (#5485)

* Fix two bugs in BigDecimal mongoization
1. demongoize - has redundant elsif object.numeric?
2. mongoize - missing to_s in return condition when map_big_decimal_to_decimal128 is false

* Remove obsolete comment

* Update big_decimal.rb

* Fix spec

* MONGOID-5507 add release notes and minor changes

Co-authored-by: shields <[email protected]>
Co-authored-by: Neil Shweky <[email protected]>

Author: johnnyshields

source/release-notes/mongoid-9.0.txt

@@ -120,3 +120,14 @@ Mongoid 9.0 flips the default of this flag from ``true`` => ``false``.
 
 This means that, by default, Mongoid 9 will update the existing document and
 will not replace it.
+
+
+Bug Fixes and Improvements
+--------------------------
+
+This section will be for smaller bug fixes and improvements:
+
+- With the ``map_big_decimal_to_decimal128`` flag set to false, ``demongoizing``
+  a non-numeric, non-string value that implements ``:to_d`` will return a string
+  rather than a ``BigDecimal``
+  `MONGOID-5507 <https://jira.mongodb.org/browse/MONGOID-5507>`_.