[7.2.245] When 'enc' is "utf-16" and 'fenc' is "utf-8" writing a file does

[7.2.245] When 'enc' is "utf-16" and 'fenc' is "utf-8" writing a file does

Problem:    When 'enc' is "utf-16" and 'fenc' is "utf-8" writing a file does
            conversion while none should be done. (Yukihiro Nakadaira) When
            'fenc' is empty the file is written as utf-8 instead of utf-16.

Solution:   Do proper comparison of encodings, taking into account that all
            Unicode values for 'enc' use utf-8 internally.

Patch 7.2.245