Avoid expensive Function.toString() in nodeLinkFn

When a directive controller is declared in Array-form (for dependency injection, good example: _NgModelController_), the method _nodeLinkFn_ shows up for us in MSIE10 profiling. The cause is apparently an expensive Function-Stringification (of e.g. NgModelController) for what should be a simple String-comparison. 

The proposed code change should solve the performance problem. It might be sufficient to simply replace `==` with `===` instead, depending on whether or not `[ '@' ]` is deemed an acceptable value for `directive.controller`.