Added condition to 1st test in cv::GaussianBlur

Consistent with the test made in cv::boxFilter, it adjusts the kernel size to the source size only if the border is not BORDER_CONSTANT and if BORDER_ISOLATED is set. Otherwise, the source has to be considered possibly in a larger image (i.e. the source being a ROI) in witch the kernel should apply.
Merge pull request #2754 from thoinvil:patch-1