Attempt to convert content to Unicode.

If set to a (byte-)string (`str` type in Python), attempt to decode it as UTF-8. Otherwise, users of libtaxii should pass in a unicode object directly.

This is necessary when passing in serialized UTF-8 XML that has been saved as a Python `str` (for instance, the output of `STIXPackage.to_xml()`, when that content contains non-ASCII characters.)