BUG: Defaults.grid_option dict instance is shared across widget instances

Environment

You won't need this. I show below the lines of code that cause the issue.
Versions

python : 3.6.7.final.0
python-bits : 64
OS : Linux
OS-release : 4.14.193-149.317.amzn2.x86_64
machine : x86_64
processor : x86_64
qgrid :1.3.1
notebook: 6.1.3
jupyterlab:2.2.0
jupyterlab-launcher:0.13.1
jupyterlab-pygments:0.1.1
jupyterlab-server:1.2.0

Jupyter lab packages

@bokeh/jupyter_bokeh v2.0.3 enabled OK
@jupyter-widgets/jupyterlab-manager v2.0.0 enabled OK
@jupyterlab/hub-extension v2.2.0 enabled OK
@pyviz/jupyterlab_pyviz v1.0.4 enabled OK
jupyterlab-drawio v0.7.0 enabled OK
qgrid2 v1.1.3 enabled OK

Description of Issue

Widgets created using show_grid without specifying grid_options share a grid_option dict instance, such that changing an option for one will change it for the others.

Reproduction Steps

This will happen for any grid_option item.

In  [2]: grid1 = qgrid.show_grid(pd.DataFrame({'a':[1]}))
         grid2 = qgrid.show_grid(pd.DataFrame({'c':[5]}))
         grid1.grid_options["maxVisibleRows"]
Out [2]: 15

In  [3]: grid2.grid_options["maxVisibleRows"]
Out [3]: 15

In  [4]: grid1.change_grid_option("maxVisibleRows", 9)
         grid1.grid_options["maxVisibleRows"]
Out [4]: 9

In  [5]: grid2.grid_options["maxVisibleRows"]
Out [5]: 9

What steps have you taken to resolve this already?

This happens because new qgrid widgets instances are assigned the default grid_options dict, not a copy of that dict, within show_grid(), if the call doesn't include an argument for grid_options

qgrid/qgrid/grid.py

Lines 484 to 493 in 877b420

    
           if grid_options is None: 
        
               grid_options = defaults.grid_options 
        
           else: 
        
               options = defaults.grid_options.copy() 
        
               options.update(grid_options) 
        
               grid_options = options 
        
           if not isinstance(grid_options, dict): 
        
               raise TypeError( 
        
                   "grid_options must be dict, not %s" % type(grid_options) 
        
               )

	if grid_options is None:
	grid_options = defaults.grid_options
	else:
	options = defaults.grid_options.copy()
	options.update(grid_options)
	grid_options = options
	if not isinstance(grid_options, dict):
	raise TypeError(
	"grid_options must be dict, not %s" % type(grid_options)
	)