Cannot threshold composite float32 images

[kephale]

To reproduce:

1. Open blobs sample image
2. Convert blobs to float32
3. In GUI: Image > Color > Merge Channels
4. Add blobs to 2 channels
5. Try to threshold resulting composite image

Observed behavior:

[imagejan]

This is not restricted to 32-bit images. In my tests, every composite image was giving this Threshold dialog, e.g.:

run("Blobs (25K)");
run("Merge Channels...", "c1=blobs.gif c2=blobs.gif c3=blobs.gif create");
run("Threshold...");