Port Ruby test case to Python

test/test-suite/test_foreign.py

@@ -128,6 +128,17 @@ class TestForeign:
         for i in range(len(before_exif)):
             assert before_exif[i] == after_exif[i]
 
+        # https://github.com/libvips/libvips/issues/1847
+        filename = temp_filename(self.tempdir, ".v")
+        x = pyvips.Image.black(16, 16) + 128
+        x.write_to_file(filename)
+
+        x = pyvips.Image.new_from_file(filename)
+        assert x.width == 16
+        assert x.height == 16
+        assert x.bands == 1
+        assert x.avg() == 128
+
         x = None
 
     @skip_if_no("jpegload")