fix double-slashes in discussion notifications

closes #CNVS-35894

Change-Id: I53b01086031bf1022f47a0e0da838a5a131913c7
Reviewed-on: https://gerrit.instructure.com/106578
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>

Author: maneframe

gems/html_text_helper/lib/html_text_helper.rb

@@ -160,8 +160,11 @@ def append_base_url(subject, base)
     tags = output.css('*[href]')
 
     tags.each do |tag|
-      next if tag.attributes['href'].value.match(/^https?|mailto|ftp/)
-      tag.attributes['href'].value = "#{base}#{tag.attributes['href']}"
+      url = tag.attributes['href'].value
+      next if url.match(/^https?|mailto|ftp/)
+
+      url.sub!("/", "") if url.start_with?("/") && base.end_with?("/")
+      tag.attributes['href'].value = "#{base}#{url}"
     end
 
     output.to_s

gems/html_text_helper/spec/html_text_helper_spec.rb

@@ -251,6 +251,13 @@ def th
 
       expect(html).to match(%r{http://example.com/relative/link})
     end
+
+    it "should resolve doubleslashes" do
+      original_html = %q{ <a href="/relative/link">Relative link</a> }
+      html          = th.html_to_simple_html(original_html, base_url: 'http://example.com/')
+
+      expect(html).to match(%r{http://example.com/relative/link})
+    end
   end
 
   context "banner" do