[meta] Use rawequal() to compare tables in isinstance().

Ensures no fancy metamethod is invoked during testing.

meta.lua

@@ -15,7 +15,7 @@ local meta = {}
 function meta.isinstance(obj, cls)
     repeat
         local m = getmetatable(obj)
-        if m == cls then return true end
+        if rawequal(m, cls) then return true end
 
         obj = m
     until obj == nil