Fix broken specs in Kiwi::Image model

Two specs were failing when checking if the subject didn't have errors:

```
Failure/Error: it { expect(subject.errors).to be_empty }
  expected `[#<Nokogiri::XML::SyntaxError: 1:12: FATAL: XML declaration allowed only at the start of the document>].empty?` to be truthy, got false
./spec/models/kiwi/image_spec.rb:226:in `block (5 levels) in <top (required)>'

Failure/Error: expect(subject.errors).to be_empty
expected `[#<Nokogiri::XML::SyntaxError: 1:12: FATAL: XML declaration allowed only at the start of the document>].empty?` to be truthy, got false
./spec/models/kiwi/image_spec.rb:279:in `block (4 levels) in <top (required)>'
```

Those errors weren't reported before nokogiri _1.13.0_, as noted in the
changelog upstream:

> XML::DocumentFragment#errors now correctly contains any parsing errors
> encountered. Previously this was always empty. (Note that
> HTML::DocumentFragment#errors already did this.

Relying on XML::DocumentFragment was wrong since the XML generated by
the Kiwi::Image's to_xml and kiwi_body methods is not a document
fragment, but a document. They contain a XML declaration (`<?xml ...?>`),
so changing the class solves the issue.

src/api/app/models/kiwi/image/xml_builder.rb

@@ -6,7 +6,7 @@ def initialize(image)
       end
 
       def build
-        doc = Nokogiri::XML::DocumentFragment.parse(@image.kiwi_body)
+        doc = Nokogiri::XML::Document.parse(@image.kiwi_body)
         image = doc.at_css('image')
 
         return nil unless image && image.first_element_child

src/api/spec/models/kiwi/image_spec.rb

@@ -221,7 +221,7 @@
           kiwi_image.save
         end
 
-        subject { Nokogiri::XML::DocumentFragment.parse(kiwi_image.to_xml) }
+        subject { Nokogiri::XML::Document.parse(kiwi_image.to_xml) }
 
         it { expect(subject.errors).to be_empty }
         it { expect(subject.xpath('.//image').length).to be(1) }
@@ -273,7 +273,7 @@
         logout
       end
 
-      subject { Nokogiri::XML::DocumentFragment.parse(kiwi_image.to_xml) }
+      subject { Nokogiri::XML::Document.parse(kiwi_image.to_xml) }
 
       it 'returns the xml for the kiwi image correctly' do
         expect(subject.errors).to be_empty
@@ -315,7 +315,7 @@
       let(:package) { create(:package) }
       let(:kiwi_image) { Kiwi::Image.build_from_xml(kiwi_xml, 'some_md5') }
 
-      subject { Nokogiri::XML::DocumentFragment.parse(kiwi_image.to_xml) }
+      subject { Nokogiri::XML::Document.parse(kiwi_image.to_xml) }
 
       before do
         allow(package).to receive(:kiwi_image_file).and_return('config.kiwi')
@@ -335,7 +335,7 @@
       let(:package) { create(:package) }
       let(:kiwi_image) { Kiwi::Image.build_from_xml(Kiwi::Image::DEFAULT_KIWI_BODY, 'some_md5') }
 
-      subject { Nokogiri::XML::DocumentFragment.parse(kiwi_image.to_xml) }
+      subject { Nokogiri::XML::Document.parse(kiwi_image.to_xml) }
 
       before do
         allow(package).to receive(:kiwi_image_file).and_return('config.kiwi')