as_ptr takes mut reference

Author: bschoenmaeckers

src/fmt.rs

@@ -69,7 +69,7 @@ impl PyUnicodeWriter {
         self.last_error.take()
     }
 
-    fn as_ptr(&self) -> *mut ffi::PyUnicodeWriter {
+    fn as_ptr(&mut self) -> *mut ffi::PyUnicodeWriter {
         self.writer.as_ptr()
     }