Where it has been agreed that an insurance upon a particular subject matter, or class of subject matter, is to be free from particular average, a marine insurer is not liable for any particular average loss not depriving the insured of the possession, at the port of destination of the whole of such subject matter or class, even though the subject matter or class becomes entirely worthless; but such insurer is liable for his proportion of all general average loss assessed upon the subject matter insured.