Wrong limit for elliptic_k on the branch but

[skirpichev]

In [1]: limit(elliptic_k(2+x), x, 0, dir=+I)
Out[1]: K(2)

In [2]: limit(elliptic_k(2+x), x, 0, dir=-I)
Out[2]: K(2)

c.f. https://reference.wolfram.com/language/ref/Limit.html:

In[1]:= Limit[EllipticK[2 + x], x -> 0, Direction -> +I]

Out[1]= EllipticK[2]

In[2]:= Limit[EllipticK[2 + x], x -> 0, Direction -> -I]

                                            5
                       (2 I) Sqrt[Pi] Gamma[-]
                                            4
Out[2]= EllipticK[2] + -----------------------
                                    3
                              Gamma[-]
                                    4

Series expansion for K(x) implemented using generalized hypergeometric function:

diofant/diofant/functions/special/elliptic_integrals.py

Lines 72 to 74 in 290922f

	def _eval_nseries(self, x, n, logx):
	from ...simplify import hyperexpand
	return hyperexpand(self.rewrite(hyper)._eval_nseries(x, n, logx))

So, we must add support for branch cuts here, see this.