Always use utf8 encoding in mysqlWriteTable.

This addresses jeffreyhorner#6 since it ensures that encoding for writing
and reading the temporary file will always match and will be sufficient to
encode all Unicode codepoints so no information will get lost.

This change is untested (since I don't have the time to set up an R package building environment just now). And of course you are free to use a completely different solution, e.g. one avoiding the temporary file altogether.