Handle MI_Metadata from CSW record XML

[benjwadams]

Handles MI_Metadata elements returned from CSW XML records, rather than returning None and causing an error later. Raises an exception when neither MD_Metadata nor MI_Metadata are found. Also sets encoding argument to 'unicode' rather than unicode (type), so that Python 3 does not break.

[amercader]

This looks good. I'll wait until the tests are fixed on master to double check is not breaking anything and I'll merge it.