Ensure the validator can handle the payload being JSON

At the moment, there's some disparity between how applications that
user Minitest and applications that use RSpec pass their payloads.

RSpec applications pass the payload as JSON and Minitest applications
pass it as a Hash.

Currently, the payload is being converted to JSON before being validated
by `JSON.fully_validate`. If the object is already JSON then .to_json is
called again. This means when JSON.parse is called on the object it does
not convert it back to a hash.

This commit updates the validator to ensure that the payload is
converted to JSON if the object passed is not a string.

lib/govuk_schemas/validator.rb

@@ -1,11 +1,13 @@
 module GovukSchemas
   class Validator
-    attr_reader :schema_name, :type, :payload
+    attr_reader :schema_name, :type
+    attr_accessor :payload
 
     def initialize(schema_name, type, payload)
       @schema_name = schema_name
       @type = type
       @payload = payload
+      ensure_payload_is_json
     end
 
     def valid?
@@ -27,20 +29,22 @@ def error_message
 
     def errors
       schema = Schema.find("#{type}_schema": schema_name)
-      validator = JSON::Validator.fully_validate(schema, payload.to_json)
+      validator = JSON::Validator.fully_validate(schema, payload)
       validator.map { |message| "- " + humanized_error(message) }.join("\n")
     end
 
     def formatted_payload
-      return payload if payload.is_a?(String)
-
-      JSON.pretty_generate(payload)
+      JSON.pretty_generate(JSON.parse(payload))
     end
 
     def humanized_error(message)
       message.gsub("The property '#/'", "The item")
              .gsub(/in schema [0-9a-f\-]+/, "")
              .strip
     end
+
+    def ensure_payload_is_json
+      self.payload = payload.to_json unless payload.is_a?(String)
+    end
   end
 end

spec/lib/validator_spec.rb

@@ -16,6 +16,13 @@
 
       expect(validator.valid?).to eq false
     end
+
+    it "handles the payload being passed as json" do
+      example = GovukSchemas::RandomExample.for_schema(publisher_schema: "placeholder").to_json
+      validator = described_class.new("placeholder", "publisher", example)
+
+      expect(validator.valid?).to eq true
+    end
   end
 
   describe "#error_message" do