BUG: avoid infinite loop for small kappa in vonmises

rho results in 0. for kappa < 1.4e-8 whch leads to nans appearing and an
infinite loop.
the second order taylor expansion is more precise up to at least 1e-5.
Closes gh-4720
Merge pull request #4751 from juliantaylor/vonmises-fix

BUG: avoid infinite loop for small kappa in vonmises