“ An insurance company provided a trip from its home office in Dallas, Texas, to New York City for a group of its agents and their wives. Rudolph and his wife were among the beneficiaries of this trip, and the Commissioner assessed its value to them as taxable income. It appears to be agreed between the parties that the tax consequences of the trip turn upon the Rudolphs' 'dominant motive and purpose' in taking the trip and the company's in offering it. ”