Make the default value for writer_options consistent

Fixes #10

Author: Hugo Osvaldo Barrera

barcode/codex.py

@@ -73,7 +73,7 @@ def build(self):
         chars.append(code39.EDGE)
         return [code39.MIDDLE.join(chars)]
 
-    def render(self, writer_options, text=None):
+    def render(self, writer_options=None, text=None):
         options = dict(module_width=MIN_SIZE, quiet_zone=MIN_QUIET_ZONE)
         options.update(writer_options or {})
         return Barcode.render(self, options, text)
@@ -252,7 +252,7 @@ def build(self):
         code += '11'
         return [code]
 
-    def render(self, writer_options, text=None):
+    def render(self, writer_options=None, text=None):
         options = dict(module_width=MIN_SIZE, quiet_zone=MIN_QUIET_ZONE)
         options.update(writer_options or {})
         return Barcode.render(self, options, text)