notify/webhook: Handle webendpoints without port (#3568)

Fixes and issue initializing webhook notification

```
FATA[0000] Initializing object layer failed              cause=Unable to initialize event \
    notification. dial tcp: missing port in address requestb.in source=[server-main.go:448:serverMain()]
```

cmd/utils.go

@@ -102,6 +102,24 @@ func urlPath2BucketObjectName(u *url.URL) (bucketName, objectName string) {
 	return bucketName, objectName
 }
 
+var portMap = map[string]string{
+	"http":  "80",
+	"https": "443",
+}
+
+// Given a string of the form "host", "host:port", or "[ipv6::address]:port",
+// return true if the string includes a port.
+func hasPort(s string) bool { return strings.LastIndex(s, ":") > strings.LastIndex(s, "]") }
+
+// canonicalAddr returns url.Host but always with a ":port" suffix
+func canonicalAddr(u *url.URL) string {
+	addr := u.Host
+	if !hasPort(addr) {
+		return addr + ":" + portMap[u.Scheme]
+	}
+	return addr
+}
+
 // checkDuplicates - function to validate if there are duplicates in a slice of endPoints.
 func checkDuplicateEndpoints(endpoints []*url.URL) error {
 	var strs []string