fix observeInput function

public/static/js/mapguesser.js

@@ -183,12 +183,23 @@ var MapGuesser = {
         document.getElementById('cover').style.visibility = 'hidden';
     },
 
-    observeInput: function (input, buttonToToggle) {
-        if (input.defaultValue !== input.value) {
-            buttonToToggle.disabled = false;
-        } else {
-            buttonToToggle.disabled = true;
+    observeInput: function (form, observedInputs) {
+        var anyChanged = false;
+
+        for (var i = 0; i < observedInputs.length; i++) {
+            var input = form.elements[observedInputs[i]];
+            if (input.type === 'checkbox') {
+                if (input.defaultChecked !== input.checked) {
+                    anyChanged = true;
+                }
+            } else {
+                if (input.defaultValue !== input.value) {
+                    anyChanged = true;
+                }
+            }
         }
+
+        form.elements['submit_button'].disabled = !anyChanged;
     },
 
     observeInputsInForm: function (form, observedInputs) {
@@ -199,19 +210,19 @@ var MapGuesser = {
                 case 'INPUT':
                 case 'TEXTAREA':
                     input.oninput = function () {
-                        MapGuesser.observeInput(this, form.elements.submit);
+                        MapGuesser.observeInput(form, observedInputs);
                     };
                     break;
                 case 'SELECT':
                     input.onchange = function () {
-                        MapGuesser.observeInput(this, form.elements.submit);
+                        MapGuesser.observeInput(form, observedInputs);
                     };
                     break;
             }
         }
 
         form.onreset = function () {
-            form.elements.submit.disabled = true;
+            form.elements['submit_button'].disabled = true;
         }
     }
 };