RGB images sent to ImageJ should use composite mode to preserve color

[ctrueden]

• Open a JPG in napari
• Send it to ImageJ using the advanced transfer button, fixing the dimensions to be correct (YXC)
• Observe that even though the dimensions are correct, the image is still in grayscale, with one channel at a time

ImageJ has a special CompositeImage class to composite multiple channels together at once, and ImageJ2 provides a hint for that, which IIRC is Dataset#setRGBMerged(true). It's unfortunate if napari-imagej needs to end up calling that—this might actually be a bugfix in PyImageJ or even in imagej-legacy somewhere—but I'm documenting the issue here because it's easy to reproduce using napari-imagej.