2d array vs 1d array inconsistency with optional argument to Plot3D

The problem is based on the way that julia distinguishes between singleton arrays and nx1 two-dimensional arrays. The optional argument `zs` for `plot3D( xs, ys, zs=zs )` is not robust to these two different representations but the first two arguments `xs, ys` are.

Say a matrix of data is generated and three sets of points for plotting are extracted from the columns of the matrix:

```
mat = [ 1 4 7;2 5 8;3 6 9 ]
x = mat[ :,1 ]
y = mat[ :,2 ]
z = mat[ :,3 ]
```

Each vector `x, y, z` is a `3-element Array{Int64,1}`. These can be plotted using:

```
using PyPlot
close( gcf() )
plot3D( x, y, zs=z )
display( gcf() )
```

However, say the matrix is transposed and the rows are spliced and transposed:

```
x = mat'[ 1,: ]'
y = mat'[ 2,: ]'
z = mat'[ 3,: ]'
```

Each vector is now a `3x1 Array{Int64,2}` and plotting as above gives:

```
using PyPlot
close( gcf() )
plot3D( x, y, zs=z )
display( gcf() )
```

```
PyError (PyObject_Call)
ValueError('could not broadcast input array from shape (3,1) into shape (3)',)
File "/Library/Python/2.7/site-packages/matplotlib-1.4.x-py2.7-macosx-10.9-intel.egg/matplotlib/backend_bases.py", line 2177, in print_figure
**kwargs)
File "/Library/Python/2.7/site-packages/matplotlib-1.4.x-py2.7-macosx-10.9-intel.egg/matplotlib/backends/backend_agg.py", line 509, in print_png
FigureCanvasAgg.draw(self)
File "/Library/Python/2.7/site-packages/matplotlib-1.4.x-py2.7-macosx-10.9-intel.egg/matplotlib/backends/backend_agg.py", line 455, in draw
self.figure.draw(self.renderer)
File "/Library/Python/2.7/site-packages/matplotlib-1.4.x-py2.7-macosx-10.9-intel.egg/matplotlib/artist.py", line 59, in draw_wrapper
draw(artist, renderer, *args, **kwargs)
File "/Library/Python/2.7/site-packages/matplotlib-1.4.x-py2.7-macosx-10.9-intel.egg/matplotlib/figure.py", line 1050, in draw
func(*args)
File "/Library/Python/2.7/site-packages/matplotlib-1.4.x-py2.7-macosx-10.9-intel.egg/mpl_toolkits/mplot3d/axes3d.py", line 275, in draw
Axes.draw(self, renderer)
File "/Library/Python/2.7/site-packages/matplotlib-1.4.x-py2.7-macosx-10.9-intel.egg/matplotlib/artist.py", line 59, in draw_wrapper
draw(artist, renderer, *args, **kwargs)
File "/Library/Python/2.7/site-packages/matplotlib-1.4.x-py2.7-macosx-10.9-intel.egg/matplotlib/axes/_base.py", line 2076, in draw
a.draw(renderer)
File "/Library/Python/2.7/site-packages/matplotlib-1.4.x-py2.7-macosx-10.9-intel.egg/mpl_toolkits/mplot3d/art3d.py", line 124, in draw
xs, ys, zs = proj3d.proj_transform(xs3d, ys3d, zs3d, renderer.M)
File "/Library/Python/2.7/site-packages/matplotlib-1.4.x-py2.7-macosx-10.9-intel.egg/mpl_toolkits/mplot3d/proj3d.py", line 198, in proj_transform
vec = vec_pad_ones(xs, ys, zs)
File "/Library/Python/2.7/site-packages/matplotlib-1.4.x-py2.7-macosx-10.9-intel.egg/mpl_toolkits/mplot3d/proj3d.py", line 187, in vec_pad_ones
vec = np.array([xs,ys,zs,np.ones(xs.shape)])

while loading In[16], in expression starting on line 4
in pyerr_check at /Users/user/.julia/v0.3/PyCall/src/exception.jl:58
in pycall at /Users/user/.julia/v0.3/PyCall/src/PyCall.jl:85
in fn at /Users/user/.julia/v0.3/PyCall/src/conversions.jl:181
in writemime at /Users/user/.julia/v0.3/pyplot/src/pyplot.jl:140
in base64 at base64.jl:125
in display_dict at /Users/user/.julia/v0.3/IJulia/src/execute_request.jl:30
in display at /Users/user/.julia/v0.3/IJulia/src/inline.jl:34
in display at multimedia.jl:150
```

Clearly Plot3D cannot handle this difference well. However, it turns out that the required arguments `xs, ys` are robust to this whereas `zs` is not. Using the `3x1 Array{Int64,2}` vectors again:

```
using PyPlot
close( gcf() )
plot3D( x, y )
display( gcf() )
```

Works fine.

Sorry for the very contrived example but this caused me some confusion earlier!
Thanks

Start at the plot3D entry point in pyplot/src/pyplot.jl and compare how xs and ys are handled with the optional zs argument. Reproduce the issue using the 1D and 3x1 examples from the report, then verify that both representations plot successfully without the matplotlib broadcasting error.

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