dmails: don't default IP addr to 127.0.0.1 in database (fixes #2908).

Bug introduced in 1400f64; that commit changed dmails so that
creator_ip_addr defaulted to CurrentUser.ip_addr like this:

    after_initialize :initialize_attributes, if: :new_record?

    def initialize_attributes
      self.from_id ||= CurrentUser.id
      self.creator_ip_addr ||= CurrentUser.ip_addr
    end

...but creator_ip_addr already defaulted to 127.0.0.1 from the database,
so the ||= assignment didn't work. Remove the database default so we
always default to CurrentUser.ip_addr.

test/unit/dmail_test.rb

@@ -6,7 +6,7 @@ class DmailTest < ActiveSupport::TestCase
       MEMCACHE.flush_all
       @user = FactoryGirl.create(:user)
       CurrentUser.user = @user
-      CurrentUser.ip_addr = "127.0.0.1"
+      CurrentUser.ip_addr = "1.2.3.4"
       ActionMailer::Base.delivery_method = :test
       ActionMailer::Base.perform_deliveries = true
       ActionMailer::Base.deliveries = []
@@ -112,6 +112,11 @@ class DmailTest < ActiveSupport::TestCase
       end
     end
 
+    should "record the creator's ip addr" do
+      dmail = FactoryGirl.create(:dmail, owner: @user)
+      assert_equal(CurrentUser.ip_addr, dmail.creator_ip_addr.to_s)
+    end
+
     should "send an email if the user wants it" do
       user = FactoryGirl.create(:user, :receive_email_notifications => true)
       assert_difference("ActionMailer::Base.deliveries.size", 1) do