Flatten "RemarkupValue" objects when setting field defaults for custom forms

Summary:
Ref T13685. "RemarkupEditField" uses a "RemarkupValue" internally, and it currently attempts to serialize into the database unsuccessfully.

Instead, flatten the value before returning it for storage.

Test Plan:
  - Edited the default description of a task in a custom form.

Maniphest Tasks: T13685

Differential Revision: https://secure.phabricator.com/D21854

src/applications/transactions/editfield/PhabricatorRemarkupEditField.php

@@ -29,6 +29,32 @@ final class PhabricatorRemarkupEditField
     return $value;
   }
 
+  public function getValueForDefaults() {
+    $value = parent::getValueForDefaults();
+
+    if ($value instanceof RemarkupValue) {
+      $value = $value->getCorpus();
+    }
+
+    return $value;
+  }
+
+  protected function getDefaultValueFromConfiguration($value) {
+
+    // See T13685. After changes to file attachment handling, the database
+    // was briefly poisoned with "array()" values as defaults.
+
+    try {
+      $value = phutil_string_cast($value);
+    } catch (Exception $ex) {
+      $value = '';
+    } catch (Throwable $ex) {
+      $value = '';
+    }
+
+    return $value;
+  }
+
   public function getMetadata() {
     $defaults = array();
 

src/view/form/control/AphrontFormTextAreaControl.php

@@ -73,7 +73,7 @@ class AphrontFormTextAreaControl extends AphrontFormControl {
 
     // NOTE: This needs to be string cast, because if we pass `null` the
     // tag will be self-closed and some browsers aren't thrilled about that.
-    $value = (string)$this->getValue();
+    $value = phutil_string_cast($this->getValue());
 
     // NOTE: We also need to prefix the string with a newline, because browsers
     // ignore a newline immediately after a <textarea> tag, so they'll eat