[plugin/log] Expand {combined} and {common} in log format (#5230)

This PR tries to address the issue raised in 5223 where `{combined}`
or `{common}` in log format will not expand when `{combined}` or `{common}`
is not the only token in the format.

This PR fixes 5223.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/log/setup.go

@@ -53,15 +53,9 @@ func logParse(c *caddy.Controller) ([]Rule, error) {
 			format := DefaultLogFormat
 
 			if strings.Contains(args[len(args)-1], "{") {
-				switch args[len(args)-1] {
-				case "{common}":
-					format = CommonLogFormat
-				case "{combined}":
-					format = CombinedLogFormat
-				default:
-					format = args[len(args)-1]
-				}
-
+				format = args[len(args)-1]
+				format = strings.Replace(format, "{common}", CommonLogFormat, -1)
+				format = strings.Replace(format, "{combined}", CombinedLogFormat, -1)
 				args = args[:len(args)-1]
 			}