Comcast shares posted modest gains Wednesday after the company announced its plan to spin off all of NBCUniversal's cable networks — except Bravo — into a separately publicly traded entity.Investors' initial shrug at the proposed transaction underscores the uncertainty of the maneuver.The hope for Comcast is that by shedding declining assets, the company's shares will rise. Cable networks are still profitable, but they're hemorrhaging subscribers and revenue every year as Americans cancel traditional pay TV ...