Use retain_all when fetching graph

[HalFrgrd]

retain_all : bool
        if True, return the entire graph even if it is not connected.
        otherwise, retain only the largest weakly connected component.

If the streets aren't all connected, some streets are not fetched. Turning retain_all on ensures all streets in the area are drawn.

[Guillaume-Fgt]

After two days scratching my head to find why some highway=raceway weren't showing, I found your PR. After checking, this was the two racetracks that weren't connected to any service road. Thank you. I solved it by editing the map in my case. It should at least be added to the Readme if this PR is not accepted.