Add interceptor test.

spec/lib/disable_email_interceptor_spec.rb

+require 'spec_helper'
+
+describe DisableEmailInterceptor do
+  before do
+    ActionMailer::Base.register_interceptor(DisableEmailInterceptor)
+  end
+
+  it 'should not send emails' do
+    Gitlab.config.gitlab.stub(:email_enabled).and_return(false)
+    expect {
+      deliver_mail
+    }.not_to change(ActionMailer::Base.deliveries, :count)
+  end
+
+  after do
+    Mail.class_variable_set(:@@delivery_interceptors, [])
+  end
+
+  def deliver_mail
+    key = create :personal_key
+    Notify.new_ssh_key_email(key.id)
+  end
+end