Added ability to generate dropdown from Enum field

Fixed the issue that `CookieCollection::has` always returns false for cookies from browser

pull

Unnecessary line (left after yiisoft@b39effd) [skip ci]

Unnecessary line (left after https://github.com/yiisoft/yii2/commit/b39effda2e3b4b1280061f490ab1f277c02e6c74)

Alternatively, if statement could be: if(substr($column->dbType, 0, 4) === 'enum'), however expected implemented option to be more robust across other database formats.

Created after followup on this issue:  https://github.com/yiisoft/yii/issues/292#issuecomment-44780779

Conflicts:
	framework/CHANGELOG.md

Conflicts:
	framework/CHANGELOG.md