Make sure Coordinates#heading follows the w3c specification.

Heading (course) can have a negative value on iOS devices when the OS is not able to determine the current heading.
This is against the w3c specification. Which only allows null and a number value >= 0 and <= 360.

See the following links for further informations:

 * https://dev.w3.org/geo/api/spec-source.html#heading
 * https://developer.apple.com/library/ios/documentation/CoreLocation/Reference/CLLocation_Class/#//apple_ref/occ/instp/CLLocation/course

www/Coordinates.js

@@ -50,7 +50,7 @@ var Coordinates = function(lat, lng, alt, acc, head, vel, altacc) {
     /**
      * The direction the device is moving at the position.
      */
-    this.heading = (head !== undefined ? head : null);
+    this.heading = (typeof head === 'number' && head >= 0 && head <= 360 ? head : null);
     /**
      * The velocity with which the device is moving at the position.
      */