Creates XML template to pass to Ares (#1131).

[bwatson78]

• app/views/catalog/show.xml.builder: stubs out template to build XML document for Ares.
• config/initializers/catalog_show_override.rb: ensures that the .xml extension looks at the builder template.
• spec/system/view_show_page_spec.rb: tests the expectations of the XML.

<?xml version="1.0" encoding="UTF-8"?>
<document>
  <title>Arntz-Bulletin; : Dokumentation der Kunst des 20. Jahrhunderts. Bibliographie, Werk-Kataloge ...</title>
  <author/>
  <edition/>
  <publisher>[Distributed in U.S. by Wittenborn, Verlag G. Arntz-Winter]</publisher>
  <publication_date>1968</publication_date>
  <isbn/>
  <issn/>
  <holdings>
    <holding>
      <barcodes type="multi-value" delim="|">010000712412|010000712414</barcodes>
      <volumes_issues type="multi-value" delim="|">V.1|V.2 PT.1-6</volumes_issues>
      <call_number>Z5935 .A762</call_number>
      <copy_number>2</copy_number>
      <library>Library Service Center</library>
      <location/>
    </holding>
  </holdings>
</document>