[ruby/ostruct] Avoid using block_given in the presence of aliases

defined?(yield) bypasses the block_given? method (or any aliases
to it) and always does the right thing.

https://github.com/ruby/ostruct/commit/4c38fe612e

lib/ostruct.rb

@@ -197,7 +197,7 @@ class OpenStruct
   #   data.each_pair.to_a   # => [[:country, "Australia"], [:capital, "Canberra"]]
   #
   def each_pair
-    return to_enum(__method__) { @table.size } unless block_given!
+    return to_enum(__method__) { @table.size } unless defined?(yield)
     @table.each_pair{|p| yield p}
     self
   end