Update to work with latest savon

Savon was refactored to fix threadsafe issues. It dups httpi object (https://github.com/savonrb/savon/commit/77cb1adf56bb1d77042505d691938464a8e58034#L0R82) before invoking supplied block therefore `#set_headers` uses original httpi object which won't be used inside savon. My patch fixes `SavonService` to change appropriate httpi object.