Add missing requires

Bundler 1.14.5 moved to lazily loading 'rubygems/spec_fetcher' which
revealed some missing requires from the JSON encoding test file.

activesupport/test/json/encoding_test_cases.rb

 require "bigdecimal"
+require "date"
+require "time"
+require "pathname"
+require "uri"
 
 module JSONTest
   class Foo