And add tests for empty string & null

app/models/site_setting.rb

@@ -129,7 +129,7 @@ class SiteSetting < ActiveRecord::Base
 
   # we need to think of a way to force users to enter certain settings, this is a minimal config thing
   setting(:notification_email, 'info@discourse.org')
-  setting(:email_custom_headers, 'Precedence: bulk|Auto-Submitted: auto-generated')
+  setting(:email_custom_headers, 'Auto-Submitted: auto-generated')
 
   setting(:allow_index_in_robots_txt, true)
 

lib/email/message_builder.rb

@@ -87,7 +87,7 @@ module Email
           value = header[1].strip
           result[name] = value if name.length > 0 && value.length > 0
         end
-      }
+      } if !string.nil?
       result
     end
 

spec/components/email/message_builder_spec.rb

@@ -84,6 +84,14 @@ describe Email::MessageBuilder do
       expect(Email::MessageBuilder.custom_headers(custom_headers_string)).to eq(custom_headers_result)
     end
 
+    it "empty headers builder" do
+      expect(Email::MessageBuilder.custom_headers("")).to eq({})
+    end
+
+    it "null headers builder" do
+      expect(Email::MessageBuilder.custom_headers(nil)).to eq({})
+    end
+
   end
 
   context "header args" do