the size of a 3D Arrow head plotted with Axes3D.quiver

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### Bug report

**Bug summary**

When plotting 3D arrows, there are no arguments in `Axes3D.quiver()` that can be used to changed the shape/size of the head of a 3D arrow. Nor is there any related attribute in `Line3DCollection` that can be used.

The size of a 3D arrow head seems to be dependent on the scale of the plot, i.e., `xlim`, `ylim`, and `zlim`. In the example below, a change in the scale of `z` causes the arrow head to "appear" or "disappear".

To make things easier to read, my observation of this issue will be posted in the first comment of this post.

**Code for reproduction**

```python
import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D

x = np.zeros(10)
y = np.zeros(10)
z = np.arange(10)*100 # remove *100 and the arrow heads will reappear.
dx = np.zeros(10)
dy = np.arange(10)
dz = np.zeros(10)

fig = plt.figure()
ax = fig.gca(projection='3d')
ax.quiver(x, y, z, dx, dy, dz)
ax.set_ylim(0,10)

plt.show()
```

**Actual outcome**


**Expected outcome**

**_Notice the difference in the scale of z axis._**
```python
import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D

x = np.zeros(10)
y = np.zeros(10)
z = np.arange(10)
dx = np.zeros(10)
dy = np.arange(10)
dz = np.zeros(10)

fig = plt.figure()
ax = fig.gca(projection='3d')
ax.quiver(x, y, z, dx, dy, dz)
ax.set_ylim(0,10)

plt.show()
```

**Matplotlib version**

* Operating system: macOS 10.13.6
* Matplotlib version: 2.2.2
* Matplotlib backend (`print(matplotlib.get_backend())`): MacOSX
* Python version: 2.7.15
* Jupyter version (if applicable): 5.6.0
* Other libraries:

matplotlib and python were installed with Anaconda.

Start with mpl_toolkits.mplot3d.Axes3D.quiver and the related Line3DCollection behavior, then run the supplied Python reproduction with both z scales. Done means the 3D arrow head can be controlled as requested and does not disappear merely because the plot's z scale changes; no specific test file is named in the issue.

Written by the indexing model from the issue text.