Reduce number of subscriptions created.
[ci skip] Document millisecond precision change in ActiveSupport::TimeWithZone#as_json
Include missing module in tag_helper

Since 6857415187810f1289068a448268264d0cf0844f we are using #safe_join to
join the content when an Array is given, so we must include the dependent
module here to make sure it's available when this module is used alone.

This was making Simple Form tests to fail with current master due to the
missing dependency.
Merge pull request #16091 from tgxworld/reduce_creation_of_subscribers

Reduce number of subscriptions created.
Merge pull request #16113 from egilburg/rails_4_1_upgrade_guide_json_time_with_zone_precision

[ci skip] Document millisecond precision change in ActiveSupport::TimeWithZone#as_json