title: longitude off by 180 degrees with cartopy Orthographic and RotatedPole
body: <p>I'm trying to place blue dots from the North pole down the prime meridian (longitude=0) but instead see the dots going down the dateline (longitude=180).
  The code:</p>
  
  <pre><code>#!/usr/bin/env python
  import matplotlib.pyplot as plt
  import cartopy.crs as ccrs
  Lon = 0
  ax = plt.axes( projection=ccrs.Orthographic( central_latitude=70,
                                               central_longitude=Lon) )
  ax.set_global()
  vector_crs = ccrs.RotatedPole( pole_latitude=90, pole_longitude=Lon )
  ax.plot([Lon, Lon, Lon, Lon, Lon, Lon],   # longitude
          [ 90,  80,  70,  60,  50,  40],   # latitude
          'bo', markersize=5, transform=vector_crs)
  ax.stock_img()
  plt.show()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/QwCYt.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/QwCYt.png" alt="enter image description here"></a></p>
  
  <p>Probably something related to the transforms but I haven't figured out what.  Cartopy version 0.14.2, Python 3.6.</p>