BUG: Indexing bugs with reordered indexes (GH6252, GH6254)

[jreback]

closes #6252
closes #6254

In [1]:  df = DataFrame(index=[3, 5, 4], columns=['A'])

In [2]: df.loc[[4, 3, 5], 'A'] = [1, 2, 3]

In [3]: df
Out[3]: 
   A
3  2
5  3
4  1

[3 rows x 1 columns]

In [4]: keys1 = ['@' + str(i) for i in range(5)]

In [5]: val1 = np.arange(5)

In [6]: keys2 = ['@' + str(i) for i in range(4)]

In [7]: val2 = np.arange(4)

In [8]: index = list(set(keys1).union(keys2))

In [9]: df = DataFrame(index = index)

In [10]: df['A'] = nan

In [11]: df.loc[keys1, 'A'] = val1

In [12]: df['B'] = nan

In [13]: df.loc[keys2, 'B'] = val2

In [14]: df
Out[14]: 
    A   B
@2  2   2
@3  3   3
@0  0   0
@1  1   1
@4  4 NaN

[5 rows x 2 columns]