[fix] zip files have "_id" instead of "id"

Allow to have either one.

src/ISTEX/Client.hs

@@ -62,7 +62,7 @@ L.makeLenses ''Document
 
 instance FromJSON Document where
   parseJSON (Object o) = do
-    _document_id       <- o .: "id"
+    _document_id       <- o .: "id" <|> o .: "_id"
     _document_title    <- o .:? "title"
     _document_authors  <- o .: "author" <|> pure []
     _document_abstract <- o .:? "abstract"