Multiple avoid problems

[tpchris1]

Problem

When I passed the avoid parameters in the distance_matrix method, it told me that it is an invalid restriction. However, I wonder if it's possible to allow multiple avoid options like avoid=tolls|highways|ferries.
Thank you!

Environment details

1. API: Distance_Matrix
2. Win 10 v2004
3. lib version: 4.4.2, Jupyter Notebook

Steps to reproduce

1. Simply add multiple avoids in the distance_matrix parameter

Code example

# example
gmaps = googlemaps.Client(key=YOUR_KEY)
gmaps.distance_matrix(origins=SOMEPLACE,destinations=SOMEPLACE, mode='driving', avoid='tolls|highways|ferries')

Stack trace

# example
D:\Program_Files\Anaconda3\lib\site-packages\googlemaps\distance_matrix.py in distance_matrix(client, origins, destinations, mode, language, avoid, units, departure_time, arrival_time, transit_mode, transit_routing_preference, traffic_model, region)
    107     if avoid:
    108         if avoid not in ["tolls", "highways", "ferries"]:
--> 109             raise ValueError("Invalid route restriction.")
    110         params["avoid"] = avoid
    111 

ValueError: Invalid route restriction.

[wangela]

I've confirmed that multiple arguments are supported by Distance Matrix API, just as they are with Directions API. Getting the documentation updated to reflect that.

I've left some review comments in #417 to hopefully get that merged soon.