zorder of drawmapboundary() gets on top of imshow()

Hi all,

I got this weird issue that changing the order of `drawmapboundary()` wrt `drawparallels()` and `drawmeridians()` gives different results. Specifically, calling `drawmapboundary(fill_color='0.8')` first leads to an overlay of grey over the `imshow()` (see left panel in figure). While putting `drawmapboundary()` after `drawparallels()` and `drawmeridians()` gives correct results (right panel in figure).


The script to create that plot:
```
import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.basemap import Basemap

def plotBmap(data, x, y, ax, order):

bmap=Basemap(projection='cyl',\
llcrnrlat=y[0],llcrnrlon=x[0],\
urcrnrlat=y[-1],urcrnrlon=x[-1],\
ax=ax)

bmap.imshow(data, interpolation='nearest')

# plot axes labels
lat_labels=np.linspace(-90, 90, 5)
lon_labels=np.linspace(0, 360, 6)
ax.set_yticks(lat_labels)
ax.set_xticks(lon_labels)
ax.tick_params(axis='both',which='major',labelsize=10)
ax.xaxis.set_ticklabels([])
ax.yaxis.set_ticklabels([])

# draw continents
bmap.drawcoastlines(linewidth=1.5,linestyle='solid',color='k',\
antialiased=True)
bmap.drawcountries(linewidth=0.5,linestyle='solid',color='k',\
antialiased=True)

if order=='before':
bmap.drawmapboundary(color='k',linewidth=1.0,fill_color='0.8')
bmap.drawparallels(lat_labels,labels=[1,1,0,0],linewidth=0,\
labelstyle='+/-',fontsize=10)
bmap.drawmeridians(lon_labels,labels=[0,0,0,1],linewidth=0,\
labelstyle='+/-',fontsize=10)
elif order=='after':
bmap.drawparallels(lat_labels,labels=[1,1,0,0],linewidth=0,\
labelstyle='+/-',fontsize=10)
bmap.drawmeridians(lon_labels,labels=[0,0,0,1],linewidth=0,\
labelstyle='+/-',fontsize=10)
bmap.drawmapboundary(color='k',linewidth=1.0,fill_color='0.8')

return bmap

if __name__=='__main__':

x=np.arange(0, 361, 1) # lon
y=np.arange(-90, 91, 1) # lat
X,Y=np.meshgrid(x,y)
z=np.sin(X/90.)+np.cos(Y/90.)

# create an area of missings
z[10:50, 20:80]=np.nan

fig, (ax1, ax2)=plt.subplots(1, 2, figsize=(12,6))

bmap1=plotBmap(z, x, y, ax1, 'before')
ax1.set_title('before')

bmap2=plotBmap(z, x, y, ax2, 'after')
ax2.set_title('after')

fig.show()
```
I guess it's the zorder that is wrong. specifying `zorder=0` in `drawmapboundary()` doesn't work, by `zordre=-1` does.

The versions of relevant packages:

* basemap = 1.2.0
* matplotlib = 2.2.3

One of the combinations that doesn't have this issue:
* basemap = 1.0.7
* matplotlib = 2.2.3

Use the provided plotBmap reproducer with Basemap 1.2.0 and Matplotlib 2.2.3, then compare its behavior with Basemap 1.0.7. Start by tracing drawmapboundary(), drawparallels(), and drawmeridians() as they create artists and z-orders. Done means drawmapboundary(fill_color='0.8') does not cover imshow() regardless of call order.

Written by the indexing model from the issue text.