Axis limits default to (0.0, 1.0, 0.0, 1.0) with a Circle patch

Consider the following code:

```
import matplotlib.pyplot as plt

fig = plt.figure()
ax = fig.gca()
circle = plt.Circle((0, 0), radius = 0.5)
ax.add_patch(circle)
print(plt.axis())
plt.show()
```

The default axis limits are printed as:

```
(0.0, 1.0, 0.0, 1.0)
```

Should this be the case?
