Nokogiri: Fix XML formatting to make tests green & remove warning

[carlosantoniodasilva]

Fix XML formatting to get Nokogiri tests passing

The mis-formatting was causing this test that was recently added to fail:

1) Error:
Recurly::AddOn::.find#test_0005_must return an addon with percentage-tiered-pricing and usage_timeframe when available:
Recurly::XML::ParseError: Recurly::XML::ParseError
  ./recurly-client-ruby/lib/recurly/xml/nokogiri.rb:7:in `rescue in initialize'
  ./recurly-client-ruby/lib/recurly/xml/nokogiri.rb:4:in `initialize'
  ./recurly-client-ruby/lib/recurly/xml.rb:61:in `initialize'
  ./recurly-client-ruby/lib/recurly/resource/pager.rb:290:in `new'
  ./recurly-client-ruby/lib/recurly/resource/pager.rb:290:in `load_from'
  ./recurly-client-ruby/lib/recurly/resource/pager.rb:171:in `load!'
  ./recurly-client-ruby/lib/recurly/resource/pager.rb:136:in `each'
  ./recurly-client-ruby/spec/recurly/add_on_spec.rb:78:in `first'
  ./recurly-client-ruby/spec/recurly/add_on_spec.rb:78:in `block (3 levels) in <top (required)>'

Fixing the format makes all tests green when running with XML=nokogiri.

Note: the XML file was added recently by #767.

Fix Nokogiri warning by passing the document as argument

This warning is triggered in multiple specs when running with
XML=nokogiri:

./recurly-client-ruby/lib/recurly/xml/nokogiri.rb:11: warning:
Passing a Node as the second parameter to Node.new is deprecated.
Please pass a Document instead, or prefer an alternative constructor
like Node#add_child. This will become an error in a future release
of Nokogiri.

By changing to passing the document instead, as per the warning message,
all tests are green and no warning is issued anymore.

Closes #754.

---

Test run

rexml

% bundle exec rake
Run options: --seed 13435

# Running:

.......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Finished in 1.462468s, 343.9391 runs/s, 607.1928 assertions/s.

503 runs, 888 assertions, 0 failures, 0 errors, 0 skips

nokogiri

% XML=nokogiri bundle exec rake
Run options: --seed 30196

# Running:

.......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Finished in 0.450451s, 1116.6586 runs/s, 1971.3576 assertions/s.

503 runs, 888 assertions, 0 failures, 0 errors, 0 skips

[douglasmiller]

Thanks for this, @carlosantoniodasilva