Fix 'endpoint' problem; avoid resampling the gridline positions.

When adding gridline labels, found that the longitude gridlines were not always appearing where intended.
Inspect lib/cartopy/tests/mpl/baseline_images/mpl/test_gridliner/gridliner1.png :
Old version had no gridline at 0deg in a global PlateCarree plot.
If you hover over the plot, the meridians occur at odd values ~ -180, -129, -77, -26, +26 ...
According to the locator attached you should be getting these values :
```
>>> import matplotlib.pyplot as plt
>>> import cartopy.crs as ccrs
>>> ax = plt.axes(projection=ccrs.PlateCarree())
>>> ax.gridlines()
<cartopy.mpl.gridliner.Gridliner object at 0x31ddad0>
>>> xlocator = ax._gridliners[0].xlocator
>>> print 'ticks for -180..+180 = ', xlocator.tick_values(-180.0, 180.0)
ticks for -180..+180 =  [-180. -120.  -60.    0.   60.  120.  180.]
>>> 
```