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Revert of r36411, as does not distinguish proc from lambda

• proc.c (rb_proc_arity): Fix Proc#arity in case of optional arguments
  [bug #5694] [rubyspec:b8b259] [rubyspec:184c8100f]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36413 b2dd03c8-39d4-4d8f-98ff-823fe69b080e