Inherit trailing_slash attribute from parent router

[Hooksie]

Resolves #115

When trailing_slash is not explicitly given on a NestedRouter interface, we copy the trailing_slash attribute from the parent.

There is some care taken to avoid just passing a True or False into the parent constructer, because it limits the ability to inherit all possible values. (Plus it avoids the need to do a roundabout truthiness inspection on the parent's trailing_slash, because the external interface is boolean instead of the internal storage which is a string.)

Had to write a little helper for the tests because URLs change across Django versions. I think I caught all the cases that matter. (It passes tox tests for all django versions using py36, which is all i ran).

[Hooksie]

amended to fix a flake8 violation

[c17r]

LGTM

[alanjds]

Thanks for the fix.