SendGrid: Fix user agent check during inbound parsing.

SendGrid changed their user agent from "SendGrid" to "Sendlib" for their
inbound POST response, which broke inbound reply-by-email parsing.

Props @boonebgorges.

See #100.

Author: r-a-y

includes/classes/bp-reply-by-email-inbound-provider-sendgrid.php

@@ -26,7 +26,7 @@ class BP_Reply_By_Email_Inbound_Provider_SendGrid extends BP_Reply_By_Email_Inbo
 	 * Webhook parser class method for SendGrid.
 	 */
 	public function webhook_parser() {
-		if ( empty( $_SERVER['HTTP_USER_AGENT'] ) || ( ! empty( $_SERVER['HTTP_USER_AGENT'] ) && 0 !== strpos( $_SERVER['HTTP_USER_AGENT'], 'SendGrid' ) ) ) {
+		if ( empty( $_SERVER['HTTP_USER_AGENT'] ) || ( ! empty( $_SERVER['HTTP_USER_AGENT'] ) && 0 !== strpos( $_SERVER['HTTP_USER_AGENT'], 'Sendlib' ) ) ) {
 			return;
 		}