Fixed `ActiveSupport::TimeWithZone#-` so precision is not unnecessarily lost

When working with objects with a nanosecond component, the `-` method may
unnecessarily cause loss of precision.

`ActiveSupport::TimeWithZone#-` should return the same result as if we were
using `Time#-`:

    Time.now.end_of_day - Time.now.beginning_of_day #=> 86399.999999999

Before:

    Time.zone.now.end_of_day.nsec #=> 999999999
    Time.zone.now.end_of_day - Time.zone.now.beginning_of_day #=> 86400.0

After:

    Time.zone.now.end_of_day - Time.zone.now.beginning_of_day
    #=> 86399.999999999
push splitting "to" up the callstack
Use null column for association key types
For our build, stick with mail 2.5.x for now

2.6 currently has many warnings, which are failing the Travis build.
Merge branch 'master' of github.com:rails/rails

* 'master' of github.com:rails/rails:
  For our build, stick with mail 2.5.x for now
  Correct result, previously showing wrong result
  Collapse PG default extractoin of most types to single regex
  Change wording of explanation about precision & scale of decimal numbers [ci skip]
  Cleaned up duplicated CHANGELOG entry [ci skip]
  reuse available belongs_to? method
  Convert StrongParameters cache to a hash. This fixes an unbounded memory leak demonstrated on @tenderlove's latest blog post:
  Partially revert deprecation of *_filter
  Pluralize params
  Add default_i18n_subject to the guides
  Fix regression on eager loading association based on SQL query rather than existing column.
  Relax mail gem constraint from ~> 2.5.4 to ~> 2.5, >= 2.5.4
  Keep column defaults in type cast form
  Return a null column when no column exists for an attribute
  Refactor XML serializer to not depend on `serialized_attributes`
  Test the serialized types of virtual columns in XML
  implement ActiveRecord::Base#pretty_print + changelog
  Remove duplicated HashWithIndifferentAccess#with_indifferent_access.
Merge pull request #15502 from sgrif/sg-use-null-column

Use null column for association key types
Revert "Correct Documentation for asset_tag_helpers"

This reverts commit c22a253d1a72602331db1eba3e91bc945eff2346.

Reason: Our documentation assumes the assets are in the proper place and
we are using the default Rails stack.

With the default Rails stack and the assets being in the correct place
the assets helpers uses the `/assets` prefix.
have an actual `NullColumn` object and update docs accordingly.

Follow up to #15438 and #15502.

/cc @sgrif
Emit suggested generator names when not found

When someone types in a generator command it currently outputs all generators. Instead we can attempt to find a subtle mis-spelling by running all generator names through a levenshtein_distance algorithm provided by rubygems. 

So now a failure looks like this:

```ruby
$ rails generate migratioooons
Could not find generator 'migratioooons'. Maybe you meant 'migration' or 'integration_test' or 'generator'
Run `rails generate --help` for more options.
```

If the suggestions are bad we leave the user with the hint to run `rails generate --help` to see all commands.
Merge pull request #15497 from schneems/schneems/levenshtein-generator-failure-messages

Emit suggested generator names when not found
Fix inverse associations test

`InverseHasManyTests#test_parent_instance_should_be_shared_within_create_block_of_new_child`
was mistakenly the same as
`InverseHasManyTests#test_parent_instance_should_be_shared_within_build_block_of_new_child`.
Update getting_started.md

It may be confusing to a Rails newbie why "Article" is suddenly being referred to with the "A" being capitalized. Thus far in the guide, the "a" has always been in lowercase. I think it is helpful to point out the reason for this change (especially since not using the capital "A" will result in an error). [ci skip]
Update getting_started.md

It's not very clear exactly what the "params" method is and does, as the current description in the guide is somewhat vague and brief. An example is helpful, especially considering this method is referenced over 30 times in this guide. [ci skip]
Merge pull request #15515 from leafac/fix-inverse-associations-block-test

Fix inverse associations test
Merge pull request #15518 from jonathancadepowers/master

Added Two Clarification "Tips" [ci skip]
Merge pull request #15421 from gchan/time_with_zone_precision

Fixed `ActiveSupport::TimeWithZone#-` so precision is not unnecessarily lost
Remove deprecated method ActiveRecord::Migrator.proper_table_name
Merge pull request #15512 from akshay-vishnoi/delete-deprecated-method

Remove deprecated method ActiveRecord::Migrator.proper_table_name