Fix getmetatable calls

Use `debug.getmetatable` instead of `getmetatable` in case there is a
`__metatable` field present in the object's metatable.

busted/context.lua

@@ -6,7 +6,7 @@ local function save()
     g[k] = rawget(_G, k)
   end
   return {
-    gmt = getmetatable(_G),
+    gmt = debug.getmetatable(_G),
     g = g,
     loaded = tablex.copy(package.loaded)
   }

busted/core.lua

@@ -24,15 +24,15 @@ local pendingMt = {
 
 local function metatype(obj)
   local otype = type(obj)
-  return otype == 'table' and (getmetatable(obj) or {}).__type or otype
+  return otype == 'table' and (debug.getmetatable(obj) or {}).__type or otype
 end
 
 local function hasToString(obj)
-  return type(obj) == 'string' or (getmetatable(obj) or {}).__tostring
+  return type(obj) == 'string' or (debug.getmetatable(obj) or {}).__tostring
 end
 
 local function isCallable(obj)
-    return (type(obj) == 'function' or (getmetatable(obj) or {}).__call)
+  return type(obj) == 'function' or (debug.getmetatable(obj) or {}).__call
 end
 
 return function()
@@ -143,7 +143,7 @@ return function()
 
   function busted.bindfenv(callable, var, value)
     local env = {}
-    local f = (getmetatable(callable) or {}).__call or callable
+    local f = (debug.getmetatable(callable) or {}).__call or callable
     setmetatable(env, { __index = getfenv(f) })
     env[var] = value
     setfenv(f, env)
@@ -152,7 +152,7 @@ return function()
   function busted.wrap(callable)
     if isCallable(callable) then
       -- prioritize __call if it exists, like in files
-      environment.wrap((getmetatable(callable) or {}).__call or callable)
+      environment.wrap((debug.getmetatable(callable) or {}).__call or callable)
     end
   end
 

busted/runner.lua

@@ -171,7 +171,7 @@ return function(options)
     local ctx = busted.context.get()
     local children = busted.context.children(ctx)
     local file = children[#children]
-    getmetatable(file.run).__call = info.func
+    debug.getmetatable(file.run).__call = info.func
   end
 
   local runs = cliArgs['repeat']