You cannot start a Plain Style Scalar with an Indicator (%). Plain

Style Scalars which have a leading % should be changed to double-quote
style scalars.

for #426

Author: isleshocky77

UPGRADE.md

@@ -77,13 +77,13 @@ a link cannot be generated):
 
     framework:
         router:
-            strict_requirements: %kernel.debug%
+            strict_requirements: "%kernel.debug%"
 
 The `default_locale` parameter is now a setting of the main `framework`
 configuration (it was under the `framework.session` in 2.0):
 
     framework:
-        default_locale: %locale%
+        default_locale: "%locale%"
 
 The `auto_start` setting under `framework.session` must be removed as it is
 not used anymore (the session is now always started on-demand). If