Avoid using block_given in the presence of aliases

[headius]

In #41 ostruct.rb was modified to not define block_given! on JRuby because it would not be recognized as being the same as block_given? and would not work properly (and we would also warn about this situation).

Unfortunately, each_pair still expects block_given! to be defined:

https://github.com/ruby/ostruct/blob/master/lib/ostruct.rb#L200

This leads to two failures running on JRuby:

Failure: test_each_pair(TC_OpenStruct):
  <#<OpenStruct name="John Smith", age=70, pension=300>>
  with id <4020> was expected to be equal? to
  <#<Enumerator: #<OpenStruct name="John Smith", age=70, pension=300>:each_pair>>
  with id <4024>.
/Users/headius/projects/ostruct/test/ostruct/test_ostruct.rb:167:in `test_each_pair'
     164:   def test_each_pair
     165:     h = {name: "John Smith", age: 70, pension: 300}
     166:     os = OpenStruct.new(h)
  => 167:     assert_same os, os.each_pair{ }
     168:     assert_equal '#<Enumerator: #<OpenStruct name="John Smith", age=70, pension=300>:each_pair>', os.each_pair.inspect
     169:     assert_equal [[:name, "John Smith"], [:age, 70], [:pension, 300]], os.each_pair.to_a
     170:     assert_equal 3, os.each_pair.size

Failure: test_initialize(TC_OpenStruct)
/Users/headius/projects/ostruct/test/ostruct/test_ostruct.rb:10:in `test_initialize'
      7:   def test_initialize
      8:     h = {name: "John Smith", age: 70, pension: 300}
      9:     assert_equal h, OpenStruct.new(h).to_h
  => 10:     assert_equal h, OpenStruct.new(OpenStruct.new(h)).to_h
     11:     assert_equal h, OpenStruct.new(Struct.new(*h.keys).new(*h.values)).to_h
     12:   end
     13: 

This can be fixed by either using the un-aliased block_given? on JRuby, or just using defined?(yield) which does not require block_given? to be available. I have done the latter here.

[headius]

@marcandre This issue is causing ostruct each_pair to be broken on JRuby, can we get a merge and release soon?

[marcandre]

Thanks for the reminder! Released as 0.5.5, thanks :-)