FIX Ensure value is in the correct format after save

[emteknetnz]

Issue #385

[GuySartorelli]

The reason SiteConfig does this and other admin sections don't is that SiteConfigLeftAndMain (and CMSProfileController which presumably would have the same problem) don't use the response negotiator.

i.e the problem goes away if we make this change in SiteConfigLeftAndMain::save_siteconfig():

+ $this->setResponse($this->getResponseNegotiator()->respond($this->getRequest()));
  $this->response->addHeader(
      'X-Status',
      rawurlencode(_t('SilverStripe\\Admin\\LeftAndMain.SAVEDUP', 'Saved.'))
  );
- return $form->forTemplate();
+ return $this->getResponse();

However, we then also need to update the validationResponseCallback defined in getEditForm. And that's still not the root cause - there's no sensible reason why rendering the form should result in adding extra brackets around the value. It speaks to a problem in the PHP code of the form field itself.

The root cause

The flow for most admin sections uses getResponseNegotiator() and renders the form fresh using getEditForm() rather than just rendering the form that came with the submission.

Calling getEditForm() fresh results in calling $form->loadDataFrom($record); which goes through MultiLinkField::setValue($value, $record) which loads the raw IDs from the relation, which do not have the '[]' brackets around the ID array.

The flow for SiteConfigLeftAndMain skips loading data from the record before rendering the form, which means it has the brackets around the value, because those brackets aren't filtered out when setSubmittedValue() is called on the field.

The fix

Add this method to MultiLinkField:

public function setSubmittedValue($value, $data = null)
{
    $value = rtrim(ltrim($value, '['), ']');
    return $this->setValue($value, $data);
}

No need for additional javascript logic.

[emteknetnz]

Have updated to do it all in PHP. There was an existing method convertValueToArray() with unit tests to handle converting values so I just expanded on it

[GuySartorelli]

#386 (comment) to be responded to

[GuySartorelli]

LGTM