Fix substitute_file functionality
Fix substitute_file functionality

After Session started proxying several calls to FileCache, the
substitute_file was not being used as expected.

alkama/racer,racer-rust/racer,dleslie/racer,sliz1/racer,jaxx/racer,phildawes/racer,pmarcelll/racer,hotpxl/racer,Wilfred/racer,kbknapp/racer,kbknapp/racer,birkenfeld/racer