betainc returns 0 instead of mpf('0.0')

When x1==x2, `betainc` returns an integer 0 instead of an mpmath object with 0 as its value. 
Found while implementing incomplete beta functions in SymPy. https://github.com/sympy/sympy/pull/20569#discussion_r572271273