Fix bug in resize_to_cover where the image is provided as a string

[brendon]

This appears to be a good fix. My original implementation always assumed image existed because we didn't use resize_to in the method name:

image_processing/lib/image_processing/processor.rb

Lines 9 to 16 in 56a839e

	if operations.dig(0, 0).to_s.start_with?("resize_") &&
	loader.empty? &&
	supports_resize_on_load?
	accumulator = source
	else
	accumulator = load_image(source, **loader)
	end

Not sure if there are any functional differences in how we load the image here and how it's loaded in processor.rb.

Probably the better fix would be to use another system for determining which methods need the image to be loaded first rather than relying on the method name.

Closes #134

[brendon]

Such bizarre test failures!