BUG: bug in multi-index where insert fails
closes https://github.com/pydata/pandas/issues/9250

on master:

```
>>> df
         3rd
1st 2nd     
a   b      0
b   d      1
>>> df.loc[('b', 'x'), '3rd'] = 2  # this works!
>>> df
         3rd
1st 2nd     
a   b      0
b   d      1
    x      2
>>> df.loc[('b', 'a'), '3rd'] = -1  # fails! sets everything to -1
>>> df
         3rd
1st 2nd     
a   b     -1
b   d     -1
    x     -1
>>> df.loc[('b', 'b'), '3rd'] = 3  # erros!
NotImplementedError: Index._join_level on non-unique index is not implemented
```

on branch:

```
>>> df
         3rd
1st 2nd     
a   b      0
b   d      1
>>> df.loc[('b', 'x'), '3rd'] = 2
>>> df.loc[('b', 'a'), '3rd'] = -1
>>> df.loc[('b', 'b'), '3rd'] = 3
>>> df
         3rd
1st 2nd     
a   b      0
b   d      1
    x      2
    a     -1
    b      3
```
