Merge pull request #206 from brackleian/master

Fix issue #168:  Force XML document declaration to be output before 'characters' event

src/writer/emitter.rs

@@ -401,6 +401,7 @@ impl Emitter {
 
     pub fn emit_characters<W: Write>(&mut self, target: &mut W,
                                       content: &str) -> Result<()> {
+        self.check_document_started(target)?;
         self.fix_non_empty_element(target)?;
         target.write_all(
             (if self.config.perform_escaping {