fix: require ostruct for OpenStruct name errors

[YOU54F]

Same error as seen in pact_broker-client pact-foundation/pact_broker-client#155

  1) creating a message pact with a string message allows a consumer to test that it can handle the expected message
     Failure/Error: message = OpenStruct.new(JSON.parse(content_string))

     NameError:
       uninitialized constant StringMessageHandler::OpenStruct
     # ./spec/features/create_message_pact_spec.rb:37:in `call'
     # ./spec/features/create_message_pact_spec.rb:89:in `block (4 levels) in <top (required)>'
     # ./lib/pact/message/consumer/consumer_contract_builder.rb:37:in `send_message_string'
     # ./spec/features/create_message_pact_spec.rb:88:in `block (3 levels) in <top (required)>'