Apply a gradient to a UIImage seems broken #23

When applying this gradient to UIImages:

image.apply(gradientColors: [UIColor.yellow, UIColor.red], locations: [1.0, 0.0])

I would expect the whole image to be covered by the gradient since the colors are completely opaque.

However, it does sometimes seem to apply a strange blend mode, although the blend mode is left to the default value (CGBlendMode.normal).

See on the following example: the gradient is wrong on the 1st image, but correct on the 2nd image.

Here is a second example with the following gradient:

image.apply(gradientColors: [UIColor.black, UIColor.black], locations: [1, 0])

The 1st image is correct but the 2nd seems to be a negative.

I am using ImageHelper.swift and ImageVIewExtension.swift from master (24581b1)