Fix worldCopyJump set Center and Zoom first

Hello,

here is the chain that produces the error in Issue #1904.
```
_onViewReset() 
-> L.Map.latLngToLayerPoint() 
-> L.Map.getPixelOrigin() 
-> L.Map._checkIfLoaded() 
-> BOOM
```

now the call of _onViewReset() is event-based and waits for the map to get ready ('load' event). That fixes Issue #1904 and should work as well for Issue #1831

jsfiddle with (manuelly) patched src.
http://jsfiddle.net/fastrde/2AuE7/8/ 

lg

fastrde