Fix undefined method `downcase' error

Contributes to
https://new-sentry.gitlab.net/organizations/gitlab/issues/483109

**Problem**

Users can provide an object type that doesn't support `downcase`
method (for example, a hash). That leads to 500 error.

**Solution**

Convert the ref_type value to string before calling `downcase` method.

Changelog: fixed

lib/extracts_ref/ref_extractor.rb

@@ -15,7 +15,7 @@ class RefExtractor
 
     class << self
       def ref_type(type)
-        return unless REF_TYPES.include?(type&.downcase)
+        return unless REF_TYPES.include?(type.to_s.downcase)
 
         type.downcase
       end

spec/lib/extracts_ref_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe ExtractsRef do
+RSpec.describe ExtractsRef, feature_category: :source_code_management do
   include described_class
   include RepoHelpers
 
@@ -98,6 +98,12 @@
 
       it { is_expected.to eq(nil) }
     end
+
+    context 'when ref_type is a hash' do
+      let(:ref_type) { { 'just' => 'hash' } }
+
+      it { is_expected.to eq(nil) }
+    end
   end
 
   it_behaves_like 'extracts refs'