Copy custom fields when converting estimate to invoice

Custom fields defined on an estimate are now carried over to the
invoice when using Convert to Invoice. Uses the same pattern as
the clone method.

Fixes #282

app/Services/EstimateService.php

@@ -358,6 +358,19 @@ class EstimateService
             $this->documentItemService->createTaxes($invoice, $estimate->taxes->toArray());
         }
 
+        if ($estimate->fields()->exists()) {
+            $customFields = [];
+
+            foreach ($estimate->fields as $data) {
+                $customFields[] = [
+                    'id' => $data->custom_field_id,
+                    'value' => $data->defaultAnswer,
+                ];
+            }
+
+            $invoice->addCustomFields($customFields);
+        }
+
         $estimate->checkForEstimateConvertAction();
 
         return Invoice::find($invoice->id);