Fixed `columns_for_distinct` of postgresql adapter
Added documentation for the :param option for resourceful routing
Swallow bundler exceptions when running tests

Let's avoid useless output when testing stuff related to Bundler.
Remove unnecessary include for integration tests.
:scissors:
Remove duplicated HashWithIndifferentAccess#with_indifferent_access.
[ci skip] Fix Compiler's Github link
Speed-up TimeWithZone to String conversion

I've noticed that `String(model.created_at)` is performing poorly in comparision
with other fields. The source of the problem is a way `Kernel#String` works: it first
tries to call `to_str` (which causes `NoMethodError` in `method_missing`) and then calls `to_s`.

Performance tests:

    tz = Time.zone.now

    Benchmark.ips do |x|
      x.report { String(tz) }
    end

Without this code:

    Calculating -------------------------------------
                               572 i/100ms
    -------------------------------------------------
                            10177.7 (±18.2%) i/s -      48620 in   5.000325s

With this code:

    Calculating -------------------------------------
                              1518 i/100ms
    -------------------------------------------------
                           138984.2 (±10.1%) i/s -     677028 in   4.974897s
Move types to the top level `ActiveRecord` namespace

`ActiveRecord::ConnectionAdapters::Type::Value` =>
`ActiveRecord::Type::Value`
Initial doc for TransactionIsolationError [ci skip]
Clear inflections after test.
Revert "Merge pull request #12778 from etehtsea/rake-default-fix"

This reverts commit ec0a2b57f67c9153cb5f7bbe2a3f66d13fe64bdd, reversing
changes made to a8bd7b16260c217290ef8fe3a166fcceac442c62.

This reverts commit 555ec36522011862c03b483c53be32410594a51e

This reverts commit 555ec36522011862c03b483c53be32410594a51e

By default, Rails uses the `default` Rake task to run all tests.
This commit changed the environment of the default task to `development`.
This clears the development database and has other negative consequences.
Merge pull request #15380 from Gaurav2728/gaurav-transaction_isolation_error

Initial doc for TransactionIsolationError [ci skip]
Formatting of content from b1db615 [ci skip]
Upgrading Ruby on Rails guide is no longer WIP [ci skip]
Merge pull request #15370 from sgrif/sg-type-namespace

Move types to the top level `ActiveRecord` namespace
Ensure custom properties work correctly with inheritance
Merge pull request #15389 from sgrif/sg-property-inheritance

Ensure custom properties work correctly with inheritance
Maintain column order when overriding existing columns

Working towards re-implementing serialized attributes to use the
properties API exposed the need for this, as serializing a column
shouldn't change the order of the columns.
Merge pull request #15390 from sgrif/sg-property-order

Maintain column order when overriding existing columns