Fix thesaurus regression from f25bdf4171

- Field class is now immutable, mergeWith() returns a new instance
- Thesaurus roots are recursively merged
- In case of field merge, Structure class do not index the previous field
- Added failing test case for previous bugs
- Added merge tests on Field class
- Added tests for all indexed stuff inside Structure class

lib/Alchemy/Phrasea/SearchEngine/Elastic/Structure/Structure.php

@@ -12,7 +12,6 @@ class Structure
     private $thesaurus_fields = array();
     private $private = array();
     private $facets = array();
-    private $aliases = array();
 
     /**
      * @param \databox[] $databoxes
@@ -34,10 +33,9 @@ class Structure
     {
         $name = $field->getName();
         if (isset($this->fields[$name])) {
-            $this->fields[$name]->mergeWith($field);
-        } else {
-            $this->fields[$name] = $field;
+            $field = $this->fields[$name]->mergeWith($field);
         }
+        $this->fields[$name] = $field;
 
         if ($field->getType() === Mapping::TYPE_DATE) {
             $this->date_fields[$name] = $field;