BUG: IntegerArray cannot cast from int to uint


```
In [25]: s = pd.Series([1, 2, 3, 4], dtype='Int64')

In [26]: s
Out[26]: 
0    1
1    2
2    3
3    4
dtype: Int64

In [27]: s.astype('UInt32')
...
TypeError: Cannot cast array from dtype('int64') to dtype('uint32') according to the rule 'same_kind'
```

with numpy dtypes this works:

```
In [30]: s = pd.Series([1, 2, 3, 4], dtype='int64')

In [31]: s.astype('uint32')
Out[31]: 
0    1
1    2
2    3
3    4
dtype: uint32
```