Avoid 3rd party helper naming collision

[billdami]

Workaround for issue caused by ember-leaftlet's inclusion of a helper called icon, which collides with the icon component property, as global helper's take precendence over property names in the ember resolver.

see: miguelcobain/ember-leaflet#175

cc @miguelcobain

[miguelcobain]

It really sucks that people need to do this in unrelated addons, but to solve this on ember-leaflet would mean introducing a breaking change (renaming the helper). Also, if I prefix the helper to leaflet-icon or something, it would probably mean that I would need to prefix all helpers. We never know what names people are using on their variables.

However, in my defense:

1. this is very easy to solve using {{this.property}} like in this PR, and with zero breaking changes for everyone
2. an Ember RFC was merged that basically deprecates {{property}} lookups, in favor of {{this.property}} lookups: Transition Away From Property Fallback Behavior emberjs/rfcs#308
3. module unification will also solve this because it will introduce namespacing for addons

[bakerac4]

@billdami can you add a comment above this line explaining why we need to do this. I know we will forgot at some point and it would be nice to have a reminder in the code

[bakerac4]

LGTM