GeoJSON layer - this context for option callbacks

[tugrul]

Following option callbacks can access GeoJSON layer
instance through this keyword.

• pointToLayer
• onEachFeature
• filter
• coordsToLatLng

[tugrul]

There are some lack about this pull request. I didn't see geometryToLayer is static function and i'm going to change this. Please ignore first commit of this pull request.

[IvanSanchez]

I cannot see why this is needed. Can you illustrate with a use case?

[tugrul]

@IvanSanchez use case https://gist.github.com/tugrul/851ed5fa3e435cf9b940

[mourner]

I don't think this is necessary, since you can just use bind to set the context whatever you need.

[tugrul]

@mourner using bind can be workaround but not solution. Also i can set option in initializer and access in this scope. Basic principle: Every callback should have context own instance.