vweb: fix parsing of form fields, send with multipart/form-data (by JS fetch)

vlib/vweb/tests/vweb_test.v

@@ -238,6 +238,19 @@ fn test_http_client_multipart_form_data() {
 	assert x.body == files[0].data
 }
 
+fn test_login_with_multipart_form_data_send_by_fetch() {
+	mut form_config := http.PostMultipartFormConfig{
+		form: {
+			'username': 'myusername'
+			'password': 'mypassword123'
+		}
+	}
+	x := http.post_multipart_form('http://${localserver}/login', form_config)!
+	assert x.status_code == 200
+	assert x.status_msg == 'OK'
+	assert x.body == 'username: xmyusernamex | password: xmypassword123x'
+}
+
 fn test_host() {
 	mut req := http.Request{
 		url: 'http://${localserver}/with_host'

vlib/vweb/tests/vweb_test_server.v

@@ -89,6 +89,11 @@ pub fn (mut app App) json_echo() vweb.Result {
 	return app.ok(app.req.data)
 }
 
+['/login'; post]
+pub fn (mut app App) login_form(username string, password string) vweb.Result {
+	return app.html('username: x${username}x | password: x${password}x')
+}
+
 ['/form_echo'; post]
 pub fn (mut app App) form_echo() vweb.Result {
 	app.set_content_type(app.req.header.get(.content_type) or { '' })