Fixes environ paths when default_locale set (#873)

If the default_locale was set and the URL path didn't contain a language code, it was behaving as if there was a language code in the URL. In that case, it was moving part of the PATH_INFO to SCRIPT_NAME, but as there wasn't any language code in the URL, it moved something else. This fixes that.

pywb/apps/frontendapp.py

@@ -667,10 +667,14 @@ def handle_request(self, environ, start_response):
             # store original script_name (original prefix) before modifications are made
             environ['ORIG_SCRIPT_NAME'] = environ.get('SCRIPT_NAME')
 
-            lang = args.pop('lang', self.default_locale)
+            lang = args.pop('lang', '')
             if lang:
                 pop_path_info(environ)
+
+            if lang:
                 environ['pywb_lang'] = lang
+            elif self.default_locale:
+                environ['pywb_lang'] = self.default_locale
 
             response = endpoint(environ, **args)