Fix A.net to resolve time when using default

[eileenmcnaughton]

Overview

Fixes failure to resolve date in authorize.net - this is likely a test-only bug.

Before

In the unit test when it is run in isolation there is no receive_date in $_REQUEST so it uses the default of 'now' however this is not correctly handled

After

Default is formatted to a date string

Technical Details

This has been passing in our test suite -but in fact it seems that it only passes because some other test is contaminating the $_REQUES

Comments

This is presumably passed through to the contribution api & hence it is not so grumpy but when
passed to the activity create fn it is failing when the test is run in isolation (& maybe other scenarios?)
due to 'now' not being parsed correctly.

Presumably it is not left blank in production scenarios as this would have triggered bug reports since it is a clear bug

[civibot]

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[seamuslee001]

This seems to make sense merging