Expand references of DocuemntStore schemas

This is a fix for #92.

Calls `expand_references!` on loaded schemas before adding them to the
`DocumentStore`.  This is intended to fix a bug where complex schemas
that include more than one level of cross-file use of `$ref`s are not
being validated correctly.

This commit includes new test cases that reproduce this issue and show
the effectiveness of the proposed fix.

The inspiration for this fix comes from a command included in the
`json_schema` gem.  In that gem, the stand-alone validator command
expands references within schemas before adding them to the store.
It seems reasonable to do the same here.

See https://github.com/brandur/json_schema/blob/20ccb82d7e18140d88ded508edd6c003865c98a0/lib/commands/validate_schema.rb#L92.

lib/json_matchers/matcher.rb

@@ -36,6 +36,10 @@ def build_and_populate_document_store
         map { |schema_path| Parser.new(schema_path).parse }.
         each { |schema| document_store.add_schema(schema) }
 
+      document_store.to_a.each do |_k, schema|
+        schema.expand_references!(store: document_store)
+      end
+
       document_store
     end
   end

spec/factories.rb

@@ -68,6 +68,7 @@
 
       initialize_with do
         FakeSchema.new(name, {
+          "id": "file:/#{name}.json#",
           "$schema": "https://json-schema.org/draft-04/schema#",
           "type": "array",
           "items": { "$ref": "file:/#{items.name}.json#" },

spec/json_matchers/match_json_schema_spec.rb

@@ -202,6 +202,24 @@
     expect(json_as_array).not_to match_json_schema(schema)
   end
 
+  it "validates against a schema that uses nested $refs" do
+    schema = create(:schema, :referencing_locations,
+                    items: create(:schema, :referencing_locations))
+    json = build(:response, :location)
+    json_as_array = [[json.to_h]]
+
+    expect(json_as_array).to match_json_schema(schema)
+  end
+
+  it "fails against a schema that uses nested $refs" do
+    schema = create(:schema, :referencing_locations,
+                    items: create(:schema, :referencing_locations))
+    json = build(:response, :invalid_location)
+    json_as_array = [[json.to_h]]
+
+    expect(json_as_array).not_to match_json_schema(schema)
+  end
+
   it "validates against a schema referencing with 'definitions'" do
     schema = create(:schema, :referencing_definitions)