Inflector as a hard dependency

https://github.com/api-platform/core/pull/2098/files#r202432943

Author: soyuka

composer.json

@@ -14,6 +14,8 @@
     ],
     "require": {
         "php": ">=7.1",
+
+        "doctrine/inflector": "^1.0",
         "psr/cache": "^1.0",
         "psr/container": "^1.0",
         "symfony/http-foundation": "^3.1 || ^4.0",