Added a check to suppress an error when $this->Form->input() is called w...

This pull is a hack to avoid a problem which cause warning when using formHelper->input with an option "'error' => false" like:
```echo $this->Form->input('field',  array ( 'error' => false) ) ;```
that causes a warning:
```Warning (2): array_merge() [function.array-merge]: Argument #1 is not an array```