Change the default `null` value for timestamps

[sgrif]

As per discussion, this changes the model generators to specify
null: false for timestamp columns. A warning is now emitted if
timestamps is called without a null option specified, so we can
safely change the behavior when no option is specified in Rails 5.

[jeremy]

Not sure how this can be safe unless old migrations are updated as well? They'll be implicitly null: true, but will generate null: false after the app is upgraded to Rails 5.

[sgrif]

That's the goal behind the deprecation warning if it is unspecified, users will need to specify null: true if they would like to keep that behavior, and will receive a deprecation warning until they specify one way or the other.

[jeremy]

The deprecation warning will come too late though, after the migration already ran on the production db.

[sgrif]

The warning would also appear any time anyone did anything related to schema.rb, including running tests after a new migration is created.

[prathamesh-sonpatki]

@sgrif @jeremy Should this be mentioned in upgrade guide?

[jschroeder9000]

This deprecation only pertains to the timestamps method that creates the created_at and updated_at columns, right? I had a migration that had both t.timestamps and t.timestamp :column_name in it spit this out at me and since the deprecation message says #timestamp (note lack of s) it was a bit confusing.