Fix freezing of @mut Objects when passing as argument
Reborrow even when passing &Object to &Object, so as to permit freezing
auto merge of #8497 : nikomatsakis/rust/improvements-to-object-coercion, r=pcwalton

While looking over the code for object coercion, I realized that it wasn't quite handling freezing and reborrowing correctly. Tweak the code, adding tests for the relevant cases. 

r? @pcwalton