add form_var_xxx_raw substitution vars (fix #1777)

When N_display exists, there is now two substitutions variables:
* form_var_xxx contains N_display
* form_var_xxx_raw contains N
Currently usefull with data_sources in ItemField (list).

wcs/formdata.py

@@ -29,11 +29,13 @@ def get_dict_with_varnames(fields, data):
     new_data = {}
     for field in fields:
         if data is not None:
-            value = data.get(field.id)
+            raw_value = data.get(field.id)
             if field.convert_value_to_str:
-                value = field.convert_value_to_str(value)
+                raw_value = field.convert_value_to_str(raw_value)
         else:
-            value = ''
+            raw_value = ''
+        display_value = data.get('%s_display' % field.id)
+        value = display_value or raw_value
         # add it as f$n$
         new_data['f%s' % field.id] = value
 
@@ -44,6 +46,8 @@ def get_dict_with_varnames(fields, data):
         # and finally add it as its manually defined variable name
         if field.varname:
             new_data['var_%s' % field.varname] = value
+            if display_value:
+                new_data['var_%s_raw' % field.varname] = raw_value
     return new_data