A bug in pandas.concat when the given dict contains None
``` python
In [2]:
import pandas

In [3]:
df0 = pandas.DataFrame([[10, 20, 30], [10, 20, 30], [10, 20, 30]])
df0
Out [3]:
    0   1   2
0  10  20  30
1  10  20  30
2  10  20  30

In [4]:
pandas.concat(dict(a=None, b=df0, c=df0[:2], d=df0[:1], e=df0))
Out [4]:
      0   1   2
a 0  10  20  30
  1  10  20  30
  2  10  20  30
b 0  10  20  30
  1  10  20  30
c 0  10  20  30
d 0  10  20  30
  1  10  20  30
  2  10  20  30
```

Checked with the current master e1129b11b1fe0016748d3d279000a2a71db8dee5 and 0.7.3
