Fix to tests._check_same unicode strings handling.

Fixes https://github.com/SciTools/iris/issues/769

Explain:
Types 'str' and 'unicode' both support method "encode"
Applying unicode.encode('utf-8') gives you a plain 'str' with escape chars coding the unicode extension characters.
You can't generally *re-encode* that, because any escapes character are themselves **invalid** characters to encode.

So when writing testfiles, only call ".encode('utf-8')" for unicode input, leave plain str data alone.