fix blobdetector

There should be an explicit convert to CV_8UC1 before passing on.
Else somebody calling the detect function from a simpleblobdetector on a 16bit image will trigger an error in the treshold function which doesn't support this.

More info: http://answers.opencv.org/question/41038/simpleblobdetector-for-16-bits-depht/?answer=41115#post-id-41115