scaleway: Fix case mismatch causing key lookup failure (#445)

The common http api client class used by the scaleway modules only
enables automatic jsonification of the request body if the
"Content-Type" header is application/json. The client only included
"Content-type" in its default set of headers (notice the case
variation).

This caused a KeyError on send() if the caller relied on the default
content-type value.

plugins/module_utils/scaleway.py

@@ -90,7 +90,7 @@ class Scaleway(object):
         self.headers = {
             'X-Auth-Token': self.module.params.get('api_token'),
             'User-Agent': self.get_user_agent_string(module),
-            'Content-type': 'application/json',
+            'Content-Type': 'application/json',
         }
         self.name = None