Add clear_credentials method to ProjectImportData

This backports changes made in
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9134.

app/models/project_import_data.rb

@@ -30,4 +30,8 @@ class ProjectImportData < ActiveRecord::Base
   def merge_credentials(hash)
     self.credentials = credentials.to_h.merge(hash) unless hash.empty?
   end
+
+  def clear_credentials
+    self.credentials = {}
+  end
 end

spec/models/project_import_data_spec.rb

@@ -39,4 +39,15 @@ describe ProjectImportData do
       expect(row.credentials).to eq({ 'number' => 10, 'foo' => 'bar' })
     end
   end
+
+  describe '#clear_credentials' do
+    it 'clears out the Hash' do
+      row = described_class.new
+
+      row.merge_credentials('number' => 10)
+      row.clear_credentials
+
+      expect(row.credentials).to eq({})
+    end
+  end
 end