Make References field singular like In-Reply-To

Closes mikel#1220

lib/mail/fields/references_field.rb

@@ -31,6 +31,10 @@ module Mail
   class ReferencesField < CommonMessageIdField #:nodoc:
     NAME = 'References'
 
+    def self.singular?
+      true
+    end
+
     def initialize(value = nil, charset = nil)
       value = value.join("\r\n\s") if value.is_a?(Array)
       super value, charset

spec/mail/message_spec.rb

@@ -165,6 +165,16 @@ def create_mail_with_splat_args
       expect(Mail::Utilities.blank?(mail.in_reply_to)).to be_truthy
     end
 
+    it "should be able to pass two In-Reply-To headers" do
+      mail = Mail.new("From: bob\r\nIn-Reply-To: <[email protected]>\r\nIn-Reply-To: <[email protected]>\r\nSubject: Hello!\r\n\r\nemail message\r\n")
+      expect(mail.in_reply_to).to eq '[email protected]'
+    end
+
+    it "should be able to pass two References headers" do
+      mail = Mail.new("From: bob\r\nReferences: <[email protected]>\r\nReferences: <[email protected]>\r\nSubject: Hello!\r\n\r\nemail message\r\n")
+      expect(mail.references).to eq '[email protected]'
+    end
+
     describe "YAML serialization" do
       before(:each) do
         # Ensure specs don't randomly fail due to messages being generated 1 second apart