text: Title: Shapefile gives warning with GSHHS feature
  Body: Hi,
  shapefile gives me a warning when I save a figure which has GSHHS features in it. Thought I'd ask it here:
  
  MWE:
  
  ```python
  import matplotlib.pyplot as plt;
  import cartopy.crs as ccrs;
  import cartopy;
  fig = plt.figure();
  proj = ccrs.Mercator(central_longitude = 0);
  ax = plt.axes(projection = proj);
  ax.add_feature(cartopy.feature.GSHHSFeature(levels = (1, 2), linewidth = 2.0));
  print("before");
  fig.savefig("test.png");
  print("after");
  ```
  output:
  
  before
  ```
  /home/max/.local/lib/python3.8/site-packages/shapefile.py:385: UserWarning: Shapefile shape has invalid polygon: no exterior rings found (must have clockwise orientation); interpreting holes as exteriors.
    warnings.warn('Shapefile shape has invalid polygon: no exterior rings found (must have clockwise orientation); interpreting holes as exteriors.')
  ```
  after
  
  What is the cause of this and how can it be resolved?
  
  OS: Ubuntu 20.04
  Cartopy: 0.18.0
  pyshp: 2.1.2
  
  Thanks in advance.