Fix WFS property format writer for multiline strings

  When updating a feature through WFS, if there are any newline
  characters in a string properties, these are lost (for example,
  geoserver handles these newlines as whitespace).

  To fix this, multiline string properties should be encoded as
  CDATA block.

src/ol/format/WFS.js

@@ -25,6 +25,7 @@ import {get as getProjection} from '../proj.js';
 import {
   readNonNegativeIntegerString,
   readPositiveInteger,
+  writeCDATASection,
   writeStringTextNode,
 } from './xsd.js';
 
@@ -938,7 +939,17 @@ function writeProperty(node, pair, objectStack) {
         GML32.prototype.writeGeometryElement(value, pair.value, objectStack);
       }
     } else {
-      writeStringTextNode(value, pair.value);
+      if (typeof pair.value === 'string') {
+        try {
+          // Generate a CDATA section to preserve whitespaces.
+          writeCDATASection(value, pair.value);
+        } catch {
+          // If value contains the CDATA closing marker ']]>', fallback to a text node.
+          writeStringTextNode(value, pair.value);
+        }
+      } else {
+        writeStringTextNode(value, pair.value);
+      }
     }
   }
 }