[FIX] base_import: Specified Date Format in Import Statment is not recognized

Steps to reproduce the bug:

1. Import a bank statement with the shown dateformad DDMMYYYY (without headers)
2. Specify this date format in the advanced options
3. Try to find the date column to assign.

Bug:

The date format is not suggested for the fields with format DDMMYYYY.

opw:769118

addons/base_import/models/base_import.py

@@ -305,6 +305,8 @@ class Import(models.TransientModel):
         if all(val.lower() in ('true', 'false', 't', 'f', '') for val in preview_values):
             return ['boolean']
         # If all values can be cast to float, type is either float or monetary
+        # Or a date/datetime if it matches the pattern
+        results = []
         try:
             thousand_separator = decimal_separator = False
             for val in preview_values:
@@ -336,7 +338,7 @@ class Import(models.TransientModel):
             if thousand_separator and not options.get('float_decimal_separator'):
                 options['float_thousand_separator'] = thousand_separator
                 options['float_decimal_separator'] = decimal_separator
-            return ['float', 'monetary']
+            results  = ['float', 'monetary']
         except ValueError:
             pass
         # Try to see if all values are a date or datetime
@@ -372,13 +374,15 @@ class Import(models.TransientModel):
         current_date_pattern = check_patterns(date_patterns, preview_values)
         if current_date_pattern:
             options['date_format'] = current_date_pattern
-            return ['date']
+            results += ['date']
 
         current_datetime_pattern = check_patterns(datetime_patterns, preview_values)
         if current_datetime_pattern:
             options['datetime_format'] = current_datetime_pattern
-            return ['datetime']
+            results += ['datetime']
 
+        if results:
+            return results
         return ['text', 'char', 'datetime', 'selection', 'many2one', 'one2many', 'many2many', 'html']
 
     @api.model
@@ -500,7 +504,7 @@ class Import(models.TransientModel):
                 'headers_type': header_types or False,
                 'preview': preview,
                 'options': options,
-                'advanced_mode': any([len(models.fix_import_export_id_paths(col)) > 1 for col in headers]),
+                'advanced_mode': any([len(models.fix_import_export_id_paths(col)) > 1 for col in headers or []]),
                 'debug': self.user_has_groups('base.group_no_one'),
             }
         except Exception, error: