Added partial days support to `DateTime`'s `advance` method.

You can now add partial days (e.g. 2.5.days) to `DateTime` with the advance method.
This was acheived by mimicing the `advance` implementation in `Time`.
Fixed changelog
WIP: pass existing joins to construct_join_dependency
Call get_routes_as_head only on HEAD requests.
Merge pull request #14834 from al2o3cr/issue14155

Correctly alias table names when joining more than once
Require active_support at the railties

Railties need to be used without having to require any dependecy first.
Merge pull request #15321 from tgxworld/only_find_routes_as_heads_for_head_request

Call get_routes_as_head only on HEAD requests.
Merge pull request #12080 from ShayDavidson/fix_datetime_partial_dates

Added partial days support to `DateTime`'s `advance` method.
Silence deprecation warning in test

Missed a case that only occured for mysql/mysql2
Merge pull request #15371 from sgrif/sg-silence-deprecation-warning

Silence deprecation warning in test