Ensure validation errors use correct formatter

Previously all validation errors returned to the client included keys
formatted in the ruby underscore style. To ensure clients receive
consistently formatted keys they should be formatted in the error 
message before responding.

Author: john-griffin

lib/jsonapi/exceptions.rb

@@ -284,6 +284,11 @@ class ValidationErrors < Error
       attr_accessor :messages
       def initialize(messages)
         @messages = messages
+        @key_formatter = JSONAPI.configuration.key_formatter
+      end
+
+      def format_key(key)
+        @key_formatter.format(key)
       end
 
       def errors
@@ -292,7 +297,7 @@ def errors
             element[1].map do |message|
               JSONAPI::Error.new(code: JSONAPI::VALIDATION_ERROR,
                                  status: :unprocessable_entity,
-                                 title: "#{element[0]} - #{message}",
+                                 title: "#{format_key(element[0])} - #{message}",
                                  detail: message,
                                  path: "/#{element[0]}")
             end

test/controllers/controller_test.rb

@@ -1662,7 +1662,7 @@ def test_create_validations_missing_attribute
     assert_equal 2, json_response['errors'].size
     assert_equal JSONAPI::VALIDATION_ERROR, json_response['errors'][0]['code']
     assert_equal JSONAPI::VALIDATION_ERROR, json_response['errors'][1]['code']
-    assert_match /date_joined - can't be blank/, response.body
+    assert_match /dateJoined - can't be blank/, response.body
     assert_match /name - can't be blank/, response.body
   end