Disable HEAD requests to info/refs

https://sentry.gitlab.net/gitlab/gitlabcom/issues/562013/

JWT::DecodeError: Nil JSON web token
  jwt.rb:120:in `decode'
    raise(JWT::DecodeError, 'Nil JSON web token') unless jwt
  gitlab/workhorse.rb:193:in `decode_jwt'
    JWT.decode(
  gitlab/workhorse.rb:189:in `verify_api_request!'
    decode_jwt(request_headers[INTERNAL_API_REQUEST_HEADER])
  workhorse_request.rb:13:in `verify_workhorse_api!'
    Gitlab::Workhorse.verify_api_request!(request.headers)
  active_support/callbacks.rb:432:in `block in make_lambda'
    lambda { |target, _, &blk| target.send filter, &blk }
...
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JWT::DecodeError: Nil JSON web token