Handle set where internalSetter and Proxy are both true

Summary:
If putComputed is called on a proxy whose target's prototype
is an array with a propname of 'length', then internalSetter will be
true, and the receiver will be a proxy.  In that case, proxy needs to
win; the behavior may assert or be UB otherwise.

Reviewed By: tmikov

Differential Revision: D23916279

fbshipit-source-id: c760356d48a02ece565fb4bc1acdafd7ccad7c68