Rails5: fix can't quote ActiveSupport::HashWithIndifferentAccess

What does this MR do?

Rails5 fix for spec/lib/gitlab/import/merge_request_creator_spec.rb. See https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/92247852.

This error was caused by trying to call connection.quote on an empty HashWithIndifferentAccess (merge_params). Since merge_params is a serialised attribute and bulk_insert only works with raw attributes, this attribute should not be in the test at all.

Rails 4 also serialises these merge_params to "'--- !ruby/hash:ActiveSupport::HashWithIndifferentAccess {}\n'".

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides