Adding an argument to the adapter for passing a block argument 

Adding an argument to the adapter for passing a block argument to the connection pool.

This allows for blocking when using threading to prevent the pool from creating more connections that the max-size allows.

Specifically was seeing the following warnings were shown when using threads to download files without the block=True:

    WARNING:requests.packages.urllib3.connectionpool:HttpConnectionPool is full, discarding connection: www.example.com