Use |to_json for matrix_synapse_push_include_content

Doing this for consistency.

Related to #117 (Github Pull Request).

roles/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -848,7 +848,7 @@ password_providers:
 # because it is loaded by the app. iPhone, however will send a
 # notification saying only that a message arrived and who it came from.
 push:
-   include_content: {{ matrix_synapse_push_include_content }}
+   include_content: {{ matrix_synapse_push_include_content|to_json }}
 
 
 # spam_checker: