Downgrading doesn't check for failed/missed payments

[dparker1005]

To re-create:

1. Payment fails at the gateway.
2. Member notified of the failed payment.
3. Member logs in and clicks the downgraded member level.
4. They’re shown the correct price and enter their new credit card details.
5. Amount charged on receipt shows $0.

Downgrading the membership level should first check that they have paid until the current date, just as the code for memberships with the same payment periods do below:

pmpro-proration/pmpro-proration.php

Lines 101 to 110 in ac569ee

	$payment_date = pmprorate_trim_timestamp( $morder->timestamp );
	$next_payment_date = pmprorate_trim_timestamp( pmpro_next_payment( $current_user->ID ) );
	$today = pmprorate_trim_timestamp( current_time( 'timestamp' ) );
	$days_in_period = ceil( ( $next_payment_date - $payment_date ) / 3600 / 24 );
	//if no days in period (next payment should have happened already) return level with no change to avoid divide by 0
	if ( $days_in_period <= 0 ) {
	return $level;
	}