Fix TypeError error on write()

Error: TypeError: write() argument 1 must be unicode, not str
The fix ensures the write is in utf-8 encoding.

Author: Marcelo Salazar

packages/mbed-greentea/mbed_greentea/mbed_greentea_cli.py

@@ -984,7 +984,7 @@ def dump_report_to_text_file(filename, content):
             """
             try:
                 with io.open(filename, encoding="utf-8", errors="backslashreplace", mode="w") as f:
-                    f.write(content)
+                    f.write(content.decode('utf-8'))
             except IOError as e:
                 gt_logger.gt_log_err("can't export to '%s', reason:"% filename)
                 gt_logger.gt_log_err(str(e))