change csv exporter to check flag inmediately instead of calling another function

scrapy/contrib/exporter/__init__.py

@@ -105,24 +105,23 @@ class CsvItemExporter(BaseItemExporter):
         self._configure(kwargs, dont_fail=True)
         self.include_headers_line = include_headers_line
         self.csv_writer = csv.writer(file, **kwargs)
-        self._headers_written = False
+        self._headers_not_written = True
 
     def export_item(self, item):
-        self._write_headers_and_set_fields_to_export(item)
+        if self._headers_not_written:
+            self._headers_not_written = False
+            self._write_headers_and_set_fields_to_export(item)
+
         fields = self._get_serialized_fields(item, default_value='', \
             include_empty=True)
         values = [x[1] for x in fields]
         self.csv_writer.writerow(values)
 
     def _write_headers_and_set_fields_to_export(self, item):
-        if self._headers_written:
-            return
-        self._headers_written = True
-        if not self.include_headers_line:
-            return
-        if not self.fields_to_export:
-            self.fields_to_export = item.fields.keys()
-        self.csv_writer.writerow(self.fields_to_export)
+        if self.include_headers_line:
+            if not self.fields_to_export:
+                self.fields_to_export = item.fields.keys()
+            self.csv_writer.writerow(self.fields_to_export)
 
 
 class PickleItemExporter(BaseItemExporter):