Fix #3669: rsolve_hyper fails to find a solution if the characteristic equation has imaginary roots
Move tests for #2943 to test_rsolve_hyper() from test_rsolve()
Merge branch 'master' into 3669-rsolve_hyper
Merge pull request #1839 from skirpichev/3669-rsolve_hyper

Fix #3669: rsolve_hyper can't find solution for F(n) + F(n - 2) + F(n - 1)==0