handlers: Handle re-direction properly for S3 requests. (#3355)

Make sure all S3 signature requests are not re-directed
to `/minio`. This should be only done for JWT and some
Anonymous requests.

This also fixes a bug found from https://github.com/bji/libs3

```
$ s3 -u list

ERROR: XmlParseFailure
```

Now after this fix shows proper output
```
$ s3 -u list
                         Bucket                                 Created
--------------------------------------------------------  --------------------
andoria                                                   2016-11-27T08:19:06Z
```

cmd/generic-handlers.go

@@ -81,8 +81,11 @@ func setBrowserRedirectHandler(h http.Handler) http.Handler {
 }
 
 func (h redirectHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	aType := getRequestAuthType(r)
+	// Re-direct only for JWT and anonymous requests coming from web-browser.
+	if aType == authTypeJWT || aType == authTypeAnonymous {
 		// Re-direction handled specifically for browsers.
-	if strings.Contains(r.Header.Get("User-Agent"), "Mozilla") && !isRequestSignatureV4(r) {
+		if strings.Contains(r.Header.Get("User-Agent"), "Mozilla") {
 			switch r.URL.Path {
 			case "/", "/webrpc", "/login", "/favicon.ico":
 				// '/' is redirected to 'locationPrefix/'
@@ -99,6 +102,7 @@ func (h redirectHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 				return
 			}
 		}
+	}
 	h.handler.ServeHTTP(w, r)
 }