Allow using closure argument in sendMail method

Allow writing:

    mailService.sendMail {
      it.from 'abc@example.com'
      it.to 'def@example.com'
      it.subject 'test'
      it.text 'some text'
    }

instead of:

    mailService.sendMail {
      from 'abc@example.com'
      to 'def@example.com'
      subject 'test'
      text 'some text'
    }

It can be sometimes more convenient and can solve problem with clashing variable names like `subject`.