Fix header link rendering when containing numbers

This fixes the problem where Markdown such as:

    ### 31st

Would get rendered as a link tag pointing to issue number 31 inside a
header tag.

See gitlab-org/gitlab-ce#14936 for more information.

lib/banzai/filter/abstract_reference_filter.rb

@@ -119,7 +119,7 @@ module Banzai
 
           elsif element_node?(node)
             yield_valid_link(node) do |link, text|
-              if ref_pattern && link =~ /\A#{ref_pattern}/
+              if ref_pattern && link =~ /\A#{ref_pattern}\z/
                 replace_link_node_with_href(node, link) do
                   object_link_filter(link, ref_pattern, link_text: text)
                 end

spec/lib/banzai/filter/issue_reference_filter_spec.rb

@@ -95,6 +95,14 @@ describe Banzai::Filter::IssueReferenceFilter, lib: true do
       result = reference_pipeline_result("Fixed #{reference}")
       expect(result[:references][:issue]).to eq [issue]
     end
+
+    it 'does not process links containing issue numbers followed by text' do
+      href = "#{reference}st"
+      doc = reference_filter("<a href='#{href}'></a>")
+      link = doc.css('a').first.attr('href')
+
+      expect(link).to eq(href)
+    end
   end
 
   context 'cross-project reference' do