Support Diactoros 2.0

This patch modifies the test requirements to allow zend-diactoros 2.0
when testing.

At this time, tests pass, which means that Expressive is already
compatible with Diactoros 2.0.0. As it's not a direct requirement, this
is simply a formality.

Author: weierophinney

composer.json

@@ -41,7 +41,7 @@
         "phpstan/phpstan-strict-rules": "^0.9",
         "phpunit/phpunit": "^7.0.1",
         "zendframework/zend-coding-standard": "~1.0.0",
-        "zendframework/zend-diactoros": "^1.7.1",
+        "zendframework/zend-diactoros": "^1.7.1 || ^2.0",
         "zendframework/zend-expressive-aurarouter": "^3.0",
         "zendframework/zend-expressive-fastroute": "^3.0",
         "zendframework/zend-expressive-zendrouter": "^3.0",