Run price & earnings through utf8_encode

This fixes the following notices (when using GBP) -

Notice: iconv() [function.iconv]: Detected an illegal character in input string in /wp-content/plugins/easy-digital-downloads/includes/admin/reporting/pdf-reports.php on line 113

Notice: iconv() [function.iconv]: Detected an illegal character in input string in /wp-content/plugins/easy-digital-downloads/includes/admin/reporting/pdf-reports.php on line 114

Also ensures that other currency symbols are displayed correctly (such as the Euro).