Remove trailing slashes from relative_url_root

Rails.configuration.relative_url_root can be set to `/` and the prefix
cleanup will fail because because the prefix is now `//assets` instead
of `/assets`

lib/premailer/rails/css_loaders/asset_pipeline_loader.rb

@@ -14,7 +14,7 @@ def load(url)
 
         def file_name(url)
           prefix = [
-            ::Rails.configuration.relative_url_root,
+            ::Rails.configuration.relative_url_root&.chomp('/'),
             ::Rails.configuration.assets.prefix,
             '/'
           ].join

spec/unit/css_loaders/asset_pipeline_loader_spec.rb

@@ -27,6 +27,16 @@
       it { is_expected.to eq('application.css') }
     end
 
+    context "when asset file path contains prefix and relative_url_root is set to root" do
+      before do
+        allow(Rails.configuration)
+          .to receive(:relative_url_root).and_return('/')
+      end
+
+      let(:asset) { '/assets/application.css' }
+      it { is_expected.to eq('application.css') }
+    end
+
     context "when asset file path contains 32 chars fingerprint" do
       let(:asset) { 'application-6776f581a4329e299531e1d52aa59832.css' }
       it { is_expected.to eq('application.css') }