JSON API 2.x compatibility

Relying on a version number is impossible because if jsonapi 2.x or 1.x is installed via a git checkout, it's impossible to determine a version number.

So we need to inspect the actual route definition.

Fortunately, to generate a URL for a route, that same route definition also needs to be retrieved. The performance cost is therefore minimal.

Of course, the result for the first generated URL also applies to all subsequent ones. So static caching is appropriate in this case (JSON API 2.x is not going to be replaced with 1.x mid-request, or vice versa).

And rather than modifying the existing method that is already pretty long, I've added a new helper method. static, to prove it is independent of everything else in the class. But most importantly, to make it trivial to remove in the future :)

Without

PHPUnit 6.5.11 by Sebastian Bergmann and contributors.

Testing Drupal\Tests\decoupled_router\Functional\DecoupledRouterFunctionalTest
E                                                                   1 / 1 (100%)

Time: 9.33 seconds, Memory: 6.00MB

There was 1 error:

1) Drupal\Tests\decoupled_router\Functional\DecoupledRouterFunctionalTest::testNegotiationNoMultilingual
Behat\Mink\Exception\ExpectationException: Current response status code is 500, but 200 expected.

/Users/wim.leers/Work/d8/vendor/behat/mink/src/WebAssert.php:770
/Users/wim.leers/Work/d8/vendor/behat/mink/src/WebAssert.php:130
/Users/wim.leers/Work/d8/modules/decoupled_router/tests/src/Functional/DecoupledRouterFunctionalTest.php:128
/Users/wim.leers/Work/d8/modules/decoupled_router/tests/src/Functional/DecoupledRouterFunctionalTest.php:147
/Users/wim.leers/Work/d8/modules/decoupled_router/tests/src/Functional/DecoupledRouterFunctionalTest.php:148

ERRORS!
Tests: 1, Assertions: 7, Errors: 1.

With

PHPUnit 6.5.11 by Sebastian Bergmann and contributors.

Testing Drupal\Tests\decoupled_router\Functional\DecoupledRouterFunctionalTest
.                                                                   1 / 1 (100%)

Time: 9.72 seconds, Memory: 6.00MB

OK (1 test, 21 assertions)

Rebased.