cdata_tostring should chain user provided __tostring

Fixes jmckaskill#38

ffi.c

@@ -2205,7 +2205,16 @@ static int cdata_tostring(lua_State* L)
 {
     struct ctype ct;
     char buf[64];
-    void* p = check_cdata(L, 1, &ct);
+    void* p;
+    int ret;
+
+    lua_settop(L, 1);
+    p = to_cdata(L, 1, &ct);
+
+    ret = call_user_op(L, "__tostring", 1, 2, &ct);
+    if (ret >= 0) {
+        return ret;
+    }
 
     if (ct.pointers == 0) {
         switch (ct.type) {

test.lua

@@ -685,6 +685,7 @@ mt.__call = function(vls, a, b) return '__call', vls.d.a .. a .. (b or 'nil')  e
 mt.__unm = function(vls) return -vls.d.a end
 mt.__concat = function(vls, a) return vls.d.a .. a end
 mt.__len = function(vls) return vls.d.a end
+mt.__tostring = function(vls) return 'string ' .. vls.d.a end
 
 vls.d.a = 5
 check(vls + 5, 10)
@@ -706,6 +707,7 @@ check(-vls, -5)
 local a,b = vls('6')
 check(a, '__call')
 check(b, '56nil')
+check(tostring(vls), 'string 5')
 
 if _VERSION ~= 'Lua 5.1' then
     check(vls .. 'str', '5str')