OFTEN-cranky Jennifer Aniston has been in an unusually cheerful mood while filming “Bounty Hunter” in Atlantic City. Credit Gerard Butler. One local spy told us, “Jennifer seems very relaxed and friendly. She has a great laugh and is very chatty on set.” The source added, “Jen and Gerard seem to have a great relationship and chemistry between them.” The two have been spotted shooting on the Boardwalk and inside the Borgata and Taj Mahal casinos, where some bellmen, doormen, valets, waitresses and dealers were cast as extras in the flick.