RUBY-2867 Change Decimal128 to deserialize to BigDecimal by default (#275)

* RUBY-2867 deserialize Decimal128 to BigDecimal by default, add tests

* RUBY-2867 add options to recursion in mri

* RUBY-2867 fix all tests

* tentative keyword arg fix

* RUBY-2867 update documentation to use present tense

* compatibility with Ruby pre-2.7

* redo parameter handling again

Co-authored-by: Oleg Pudeyev <code@olegp.name>

Author: neilshweky

docs/tutorials/bson-v4.txt

@@ -549,7 +549,7 @@ follows:
 
 .. note::
 
-  In BSON 5.0, ``BSON::Decimal128`` will be deserialized into ``BigDecimal`` by
+  In BSON 5.0, ``BSON::Decimal128`` is deserialized into ``BigDecimal`` by
   default. In order to have ``BSON::Decimal128`` values in BSON documents
   deserialized into ``BSON::Decimal128``, the ``mode: :bson`` option can be set
   on ``from_bson``.