Improve work with weak keys.

```
for _, m in ipairs({'', 'k', 'kv', 'v'}) do
	print('test', m)
	a = {}; setmetatable(a, {__mode = m});
	a[1], a[2], a[3] = {}, {}, {};
	for k, v in pairs(a) do
		print(k, v)
	end
end
print('ok')
```

src/core/org/luaj/vm2/LuaTable.java

@@ -472,7 +472,7 @@ public class LuaTable extends LuaValue implements Metatable {
 				}
 			}
 			if ( checkLoadFactor() ) {
-				if ( (m_metatable == null || !(m_metatable.useWeakKeys() || m_metatable.useWeakValues()))
+				if ( (m_metatable == null || !m_metatable.useWeakValues())
 					&& key.isinttype() && key.toint() > 0 ) {
 					// a rehash might make room in the array portion for this key.
 					rehash( key.toint() );