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

composer.json

@@ -83,7 +83,8 @@
         "twig/twig":                             "~1.14, >=1.14.2",
         "vierbergenlars/php-semver":             "~2.1",
         "willdurand/negotiation":                "~1.3",
-        "zend/gdata":                            "~1.12.1"
+        "zend/gdata":                            "~1.12.1",
+        "beberlei/assert": "2.3"
     },
     "require-dev":  {
         "phpunit/phpunit": "~4.5",