Shania Twain spent her birthday weekend doing what she loves most — performing on stage. The singer turned 59 years old on Wednesday (Aug. 28), and during her Las Vegas residency date scheduled for the following night, she held a mini-birthday celebration onstage, treating fans to a surprise appearance from legendary R&B group Boyz II Men. But what Twain didn’t see coming was that the group was planning a special surprise of their own. The two acts gave a duet rendition of Twain’s late-’90s hit “You’re Still the One,” followed by an a capella performance of Boyz II Men’s classic “I’ll Make Lov…