Use encode! rather than encode to modify string in place

app/models/marc_collection_document_callbacks.rb

@@ -25,7 +25,7 @@ def start_element(name, attrs = [])
 
   # Write any text that can be found in an open tag
   def characters(string)
-    @io.write string.encode(xml: :text)
+    @io.write string.encode!(xml: :text)
   end
 
   # Write the closing tag for an element

spec/models/marc_collection_document_callbacks_spec.rb

@@ -25,7 +25,9 @@
 
   describe '#characters' do
     it "escapes special XML characters" do
-      document_callbacks.characters('Vols. for 1972-<1982> called also vyp. 1-<8/2>.')
+      # Nokogiri's SAX parser gives unfrozen strings to this callback, so we use the +
+      # in this test to make sure the string we pass is similarly unfrozen
+      document_callbacks.characters(+'Vols. for 1972-<1982> called also vyp. 1-<8/2>.')
       expect(io.string).to eq('Vols. for 1972-&lt;1982&gt; called also vyp. 1-&lt;8/2&gt;.')
     end
   end