Behaviour of c=None is inconsistent in 2.0

The following code
```python
import numpy as np
import matplotlib.pyplot as plt
f,ax = plt.subplots()
x = np.arange(10)
y = 2*x

plt.errorbar(x,y, c=None)
plt.errorbar(x,2*y, c=None)
```

Produces two lines with the same color (first one of the cycle). In 1.5 it is two different colors.
It is also two colors if I change `errorbar` to `plot`. 