Support of array of @attributes (tests ok)

on upstream/master it acts unexpectedly
see tests on upstream
```ruby
..............................FF..............................

Failures:

  1) Gyoku::Hash.to_xml recognizes array of attributes
     Failure/Error: to_xml(hash).should == '<category name="one"></category><category name="two"></category>'
       expected: "<category name=\"one\"></category><category name=\"two\"></category>"
            got: "<category></category><category></category>" (using ==)
     # ./spec/gyoku/hash_spec.rb:214:in `block (3 levels) in <top (required)>'

  2) Gyoku::Hash.to_xml recognizes array of attributes with selfclosing tag
     Failure/Error: to_xml(hash).should == '<category name="one"/><category name="two"/>'
       expected: "<category name=\"one\"/><category name=\"two\"/>"
            got: "<category></category><category></category>" (using ==)
     # ./spec/gyoku/hash_spec.rb:221:in `block (3 levels) in <top (required)>'

Finished in 0.0277 seconds
62 examples, 2 failures
```
this fixes it