[BUGFIX] Change check on Asset "path" to cover 4.5 misbehavior

Instead of using isset() which always returns TRUE for the ArrayAccess used in 4.5's Fluid, the check needs to be extended.

Replaces: #150
Merge pull request #158 from NamelessCoder/pathcheck

[BUGFIX] Change check on Asset "path" to cover 4.5 misbehavior