Non-set keys should not be altered

Moved https://github.com/cakephp/migrations/issues/330 over to here.

Currently

		$table->changeColumn('status', 'string', [
			'encoding' => 'utf8mb4',
			'collation' => 'utf8mb4_unicode_ci',
		]);

Kills most other settings like

			'null' => true,
			'default' => null,

Silently changing the other table field properties here now makes inserts impossible without specifying these now non-null-default values.

One always needs to check and add all those into every changeColumn, instead of just altering what is defined here in the migration file.
This leads to some really dangerous DB regressions.
Maybe we can merge with existing scheme before applying?

Start at the changeColumn entry point shown in the issue and reproduce the migration that changes only the column type while existing null and default settings are present. Trace how the current column definition is built and applied, then verify that unspecified properties remain unchanged and that inserts still work without explicitly supplying them.