FIX: Respect protocol relative URLs in email styling

lib/email/styles.rb

@@ -20,7 +20,7 @@ module Email
           img['style'] = "max-width: 694px;"
         end
 
-        if img['src'][0] == "/"
+        if img['src'] =~ /^\/[^\/]/
           img['src'] = "#{Discourse.base_url}#{img['src']}"
         end
       end

spec/components/email/styles_spec.rb

@@ -40,6 +40,11 @@ describe Email::Styles do
       expect(frag.at("img")["src"]).to eq("#{Discourse.base_url}/some-image.png")
     end
 
+    it "it does not change protocol relative paths" do
+      frag = basic_fragment("<img src='//www.discourse.com/some-image.gif'>")
+      expect(frag.at("img")["src"]).to eq("//www.discourse.com/some-image.gif")
+    end
+
     it "strips classes and ids" do
       frag = basic_fragment("<div class='foo' id='bar'><div class='foo' id='bar'></div></div>")
       expect(frag.to_html).to eq("<div><div></div></div>")