Create assert_response_schema test helper

It is a common pattern to use JSON Schema to validate a API response[1], [2]
and [3].

This patch creates the `assert_response_schema` test helper that helps people do
this kind of validation easily on the controller tests.

[1]: https://robots.thoughtbot.com/validating-json-schemas-with-an-rspec-matcher
[2]: https://github.com/sharethrough/json-schema-rspec
[3]: https://github.com/rails-api/active_model_serializers/issues/1011#issuecomment-127608121

test/support/schemas/active_model_serializers/test/schema_test/my/index.json

@@ -0,0 +1,6 @@
+{
+  "properties": {
+    "name" : { "type" : "string" },
+    "description" : { "type" : "string" }
+  }
+}

test/support/schemas/active_model_serializers/test/schema_test/my/show.json

@@ -0,0 +1,6 @@
+{
+  "properties": {
+    "name" : { "type" : "integer" },
+    "description" : { "type" : "boolean" }
+  }
+}

test/support/schemas/custom/show.json

@@ -0,0 +1,7 @@
+{
+  "id": "file://custom/show.json#",
+  "properties": {
+    "name" : { "type" : "string" },
+    "description" : { "type" : "string" }
+  }
+}

test/support/schemas/hyper_schema.json

@@ -0,0 +1,93 @@
+{
+  "$schema": "http://json-schema.org/draft-04/hyper-schema",
+  "title": "Profile",
+  "description": "Profile schema",
+  "stability": "prototype",
+  "strictProperties": true,
+  "type": [
+    "object"
+  ],
+  "definitions": {
+    "name": {
+      "description": "unique name of profile",
+      "readOnly": true,
+      "type": [
+        "string"
+      ]
+    },
+    "description": {
+      "description": "description of profile",
+      "readOnly": true,
+      "type": [
+        "string"
+      ]
+    },
+    "identity": {
+      "anyOf": [
+        {
+          "$ref": "/schemata/profile#/definitions/name"
+        }
+      ]
+    }
+  },
+  "links": [
+    {
+      "description": "Create a new profile.",
+      "href": "/profiles",
+      "method": "POST",
+      "rel": "create",
+      "schema": {
+        "properties": {
+        },
+        "type": [
+          "object"
+        ]
+      },
+      "title": "Create"
+    },
+    {
+      "description": "Delete an existing profile.",
+      "href": "/profiles/{(%2Fschemata%2Fprofile%23%2Fdefinitions%2Fidentity)}",
+      "method": "DELETE",
+      "rel": "destroy",
+      "title": "Delete"
+    },
+    {
+      "description": "Info for existing profile.",
+      "href": "/profiles/{(%2Fschemata%2Fprofile%23%2Fdefinitions%2Fidentity)}",
+      "method": "GET",
+      "rel": "self",
+      "title": "Info"
+    },
+    {
+      "description": "List existing profiles.",
+      "href": "/profiles",
+      "method": "GET",
+      "rel": "instances",
+      "title": "List"
+    },
+    {
+      "description": "Update an existing profile.",
+      "href": "/profiles/{(%2Fschemata%2Fprofile%23%2Fdefinitions%2Fidentity)}",
+      "method": "PATCH",
+      "rel": "update",
+      "schema": {
+        "properties": {
+        },
+        "type": [
+          "object"
+        ]
+      },
+      "title": "Update"
+    }
+  ],
+  "properties": {
+    "name": {
+      "$ref": "/schemata/profile#/definitions/name"
+    },
+    "description": {
+      "$ref": "/schemata/profile#/definitions/description"
+    }
+  },
+  "id": "/schemata/profile"
+}

test/support/schemas/render_using_json_api.json

@@ -0,0 +1,43 @@
+{
+  "$schema": "http://json-schema.org/draft-04/schema#",
+  "id": "",
+  "type": "object",
+  "properties": {
+    "data": {
+      "id": "/data",
+      "type": "object",
+      "properties": {
+        "id": {
+          "id": "/data/id",
+          "type": "string"
+        },
+        "type": {
+          "id": "/data/type",
+          "type": "string"
+        },
+        "attributes": {
+          "id": "/data/attributes",
+          "type": "object",
+          "properties": {
+            "name": {
+              "id": "/data/attributes/name",
+              "type": "string"
+            },
+            "description": {
+              "id": "/data/attributes/description",
+              "type": "string"
+            }
+          }
+        }
+      },
+      "required": [
+        "id",
+        "type",
+        "attributes"
+      ]
+    }
+  },
+  "required": [
+    "data"
+  ]
+}

test/support/schemas/simple_json_pointers.json

@@ -0,0 +1,10 @@
+{
+  "properties": {
+    "name": {
+      "$ref": "file://custom/show.json#/properties/name"
+    },
+    "description": {
+      "$ref": "file://custom/show.json#/properties/description"
+    }
+  }
+}