Set content-type when signaling deployment

In signal_deployments of
ParallelDeploymentsTest.test_deployments_metadata, set
the content-type to 'application/json'.

Change-Id: Id0ac6d76545bad1857aab2f85b65db2101df655e
Closes-Bug: #1656613
diff --git a/functional/test_software_config.py b/functional/test_software_config.py
index 860d688..815893f 100644
--- a/functional/test_software_config.py
+++ b/functional/test_software_config.py
@@ -149,7 +149,7 @@
             iv = dict((i['name'], i['value']) for i in dep['inputs'])
             sigurl = iv.get('deploy_signal_id')
             requests.post(sigurl, data='{}',
-                          headers={'content-type': None},
+                          headers={'content-type': 'application/json'},
                           verify=self.verify_cert)