Ensure X-Designate-Edit-Managed-Records header value is a string

Header values are strings so we should provide a string as
X-Designate-Edit-Managed-Records header value instead of a boolean
(which is transformed into: "<type 'bool'>" by python-requests).

Change-Id: Iaf248a0e817fe65785ddcdca46663e7273a6dad0

designateclient/v2/client.py

@@ -64,7 +64,7 @@ class DesignateAdapter(adapter.LegacyJsonAdapter):
         if self.edit_managed:
             kwargs['headers'].setdefault(
                 'X-Designate-Edit-Managed-Records',
-                self.edit_managed
+                str(self.edit_managed)
             )
 
         if self.sudo_project_id is not None: