Fix one incorrect mapping where utf-8 should be utf-16

Plus add 3 more mappings for utf8mb4 307,308,309

Author: testn

lib/constants/charset_encodings.js

@@ -104,7 +104,7 @@ module.exports = [
   'eucjpms',
   'eucjpms',
   'cp1250',
-  'utf8',
+  'utf16',
   'utf16',
   'utf16',
   'utf16',
@@ -309,5 +309,8 @@ module.exports = [
   'utf8',
   'utf8',
   'utf8',
+  'utf8',
+  'utf8',
+  'utf8',
   'utf8'
 ];