LState.CheckString should cast number to string and return

[cc682]

LState.CheckString will fail when passed a number param, this is different with lua 5.1.This is the document from lua.org:

luaL_checklstring
const char *luaL_checklstring (lua_State *L, int narg, size_t *l);
Checks whether the function argument narg is a string and returns this string; if l is not NULL fills *l with the string's length.

This function uses lua_tolstring to get its result, so all conversions and caveats of that function apply here.

[coveralls]

Coverage increased (+0.003%) to 88.117% when pulling a519e4d on cc682:master into ab39c60 on yuin:master.