Regression test failure: CCDData.meta.setter

[MSeifert04]

There is a test failure because of the newly merged astropy/astropy#4509

It's pretty easy to fix:

• Delete meta.setter and rely on NDData
• Change meta.setter to use an OrderedDict (instead of {}) if None

Actually I find it rather strange that (before the newest astropy-master) during init meta defaults to OrderedDict but if manually set on the attribute it defaults to dict. Was there any specific reason?