Extra column in csv export when sponsorship_total >= 1000

Bug introduced by commit 169b882.

Due to the use of number_format() function with default parameters, PHP
outputs the sponsorship amount formatted with ',' as a thousands
separator, which also happens to be MantisBT's default csv separator.
Since the fields are not escaped, this causes an extra column to be
generated.

Call to number_format() was removed, as it makes more sense to output
raw data in csv exports.

Fixes #13958

core/csv_api.php

@@ -420,5 +420,5 @@ function csv_format_due_date( $p_bug ) {
  * @access public
  */
 function csv_format_sponsorship_total( $p_bug ) {
-	return number_format( $p_bug->sponsorship_total );
+	return csv_escape_string( $p_bug->sponsorship_total );
 }