math.c: C99-like atan2

* math.c (math_atan2): return values like as expected by C99 if
  both two arguments are infinity.  based on the patch by cremno
  phobia <cremno AT mail.ru> in [ruby-core:62310].  [Feature #9799]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45805 b2dd03c8-39d4-4d8f-98ff-823fe69b080e