fix($compile): handle boolean attributes in @ bindings

Commit db5e0ff handles initial values of boolean attributes. The same
change needs to be applied inside the attrs.$observe() call.

Author: Jan Niehusmann

src/ng/compile.js

@@ -2994,7 +2994,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
               destination[scopeName] = attrs[attrName] = void 0;
             }
             attrs.$observe(attrName, function(value) {
-              if (isString(value)) {
+              if (isString(value) || isBoolean(value)) {
                 destination[scopeName] = value;
               }
             });