Add new 'declaration' option to control generation of an XML declarat…

…ion, and default to generating a standard one.

README.md

@@ -77,7 +77,7 @@ When writing files the API accepts several options:
 * `path`: Location to write files.
 * `rowTag`: The row tag of your xml files to treat as a row. For example, in `<books> <book><book> ...</books>`, the appropriate value would be `book`. Default is `ROW`.
 * `rootTag`: The root tag of your xml files to treat as the root. For example, in `<books> <book><book> ...</books>`, the appropriate value would be `books`. It can include basic attributes by specifying a value like `books foo="bar"` (as of 0.11.0). Default is `ROWS`.
-* `declaration`: Content of XML declaration to write at the start of every output XML file, before the `rootTag`. For example, a value of `foo` causes `<?xml foo?>` to be written. Set to null or empty to suppress. Defaults to `version="1.0" encoding="UTF-8"`. New in 0.14.0.
+* `declaration`: Content of XML declaration to write at the start of every output XML file, before the `rootTag`. For example, a value of `foo` causes `<?xml foo?>` to be written. Set to null or empty to suppress. Defaults to `version="1.0" encoding="UTF-8" standalone="yes"`. New in 0.14.0.
 * `nullValue`: The value to write `null` value. Default is string `null`. When this is `null`, it does not write attributes and elements for fields.
 * `attributePrefix`: The prefix for attributes so that we can differentiating attributes and elements. This will be the prefix for field names. Default is `_`.
 * `valueTag`: The tag used for the value when there are attributes in the element having no child. Default is `_VALUE`.

src/main/scala/com/databricks/spark/xml/XmlOptions.scala

@@ -72,7 +72,7 @@ private[xml] object XmlOptions {
   val DEFAULT_VALUE_TAG = "_VALUE"
   val DEFAULT_ROW_TAG = "ROW"
   val DEFAULT_ROOT_TAG = "ROWS"
-  val DEFAULT_DECLARATION = "version=\"1.0\" encoding=\"UTF-8\""
+  val DEFAULT_DECLARATION = "version=\"1.0\" encoding=\"UTF-8\" standalone=\"yes\""
   val DEFAULT_CHARSET: String = StandardCharsets.UTF_8.name
   val DEFAULT_NULL_VALUE: String = null
   val DEFAULT_WILDCARD_COL_NAME = "xs_any"

src/test/scala/com/databricks/spark/xml/XmlSuite.scala

@@ -385,7 +385,8 @@ final class XmlSuite extends AnyFunSuite with BeforeAndAfterAll {
       .options(Map("rootTag" -> "books", "rowTag" -> "book"))
       .xml(copyFilePath2.toString)
 
-    assert(getLines(copyFilePath2.resolve("part-00000")).head ==="<?xml version=\"1.0\" encoding=\"UTF-8\"?>")
+    assert(getLines(copyFilePath2.resolve("part-00000")).head ===
+      "<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"yes\"?>")
   }
 
   test("DSL save with nullValue and treatEmptyValuesAsNulls") {