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

@@ -81,22 +81,26 @@ func setBrowserRedirectHandler(h http.Handler) http.Handler {
 }
 
 func (h redirectHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	// Re-direction handled specifically for browsers.
-	if strings.Contains(r.Header.Get("User-Agent"), "Mozilla") && !isRequestSignatureV4(r) {
-		switch r.URL.Path {
-		case "/", "/webrpc", "/login", "/favicon.ico":
-			// '/' is redirected to 'locationPrefix/'
-			// '/webrpc' is redirected to 'locationPrefix/webrpc'
-			// '/login' is redirected to 'locationPrefix/login'
-			location := h.locationPrefix + r.URL.Path
-			// Redirect to new location.
-			http.Redirect(w, r, location, http.StatusTemporaryRedirect)
-			return
-		case h.locationPrefix:
-			// locationPrefix is redirected to 'locationPrefix/'
-			location := h.locationPrefix + "/"
-			http.Redirect(w, r, location, http.StatusTemporaryRedirect)
-			return
+	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") {
+			switch r.URL.Path {
+			case "/", "/webrpc", "/login", "/favicon.ico":
+				// '/' is redirected to 'locationPrefix/'
+				// '/webrpc' is redirected to 'locationPrefix/webrpc'
+				// '/login' is redirected to 'locationPrefix/login'
+				location := h.locationPrefix + r.URL.Path
+				// Redirect to new location.
+				http.Redirect(w, r, location, http.StatusTemporaryRedirect)
+				return
+			case h.locationPrefix:
+				// locationPrefix is redirected to 'locationPrefix/'
+				location := h.locationPrefix + "/"
+				http.Redirect(w, r, location, http.StatusTemporaryRedirect)
+				return
+			}
 		}
 	}
 	h.handler.ServeHTTP(w, r)