Add bigdecimal gem for test

the activesupport gem depends on the base64 gem, which has been extracted as a bundled gem since Ruby 3.4.
Let's install the base64 gem only for test.

Author: a2ikm

Gemfile

@@ -6,6 +6,9 @@ gem 'rake'
 if RUBY_VERSION >= '3.4'
   # the mongo gem depends on the base64 gem, which has been extracted as a bundled gem since Ruby 3.4.
   gem 'base64'
+
+  # the activesupport gem depends on the base64 gem, which has been extracted as a bundled gem since Ruby 3.4.
+  gem 'bigdecimal'
 end
 
 group(:test) do

Gemfile.lock

@@ -8,6 +8,7 @@ GEM
   remote: https://rubygems.org/
   specs:
     base64 (0.2.0)
+    bigdecimal (3.1.9)
     bson (5.0.2)
     byebug (11.1.3)
     coderay (1.1.3)
@@ -76,6 +77,7 @@ PLATFORMS
 
 DEPENDENCIES
   base64
+  bigdecimal
   byebug
   guard
   guard-bundler