Fix rabbitmq_exchange create command

Without this patch, the rabbitmq_exchange provider tries to call
rabbitmqadmin with the optional parameters internal, auto_delete, and
durable parameters set to empty, which results in an error:
`[u'not_boolean', u'']`. This patch changes the command to only use
call those parameters if they were specified by the user, otherwise the
default values will be used.

lib/puppet/provider/rabbitmq_exchange/rabbitmqadmin.rb

@@ -98,7 +98,12 @@ def create
     if arguments.nil?
       arguments = {}
     end
-    rabbitmqadmin('declare', 'exchange', vhost_opt, "--user=#{resource[:user]}", "--password=#{resource[:password]}", "name=#{name}", "type=#{resource[:type]}", "internal=#{resource[:internal]}", "durable=#{resource[:durable]}", "auto_delete=#{resource[:auto_delete]}", "arguments=#{arguments.to_json}", '-c', '/etc/rabbitmq/rabbitmqadmin.conf')
+    cmd = ['declare', 'exchange', vhost_opt, "--user=#{resource[:user]}", "--password=#{resource[:password]}", "name=#{name}", "type=#{resource[:type]}",]
+    cmd << "internal=#{resource[:internal]}" if resource[:internal]
+    cmd << "durable=#{resource[:durable]}" if resource[:durable]
+    cmd << "auto_delete=#{resource[:auto_delete]}" if resource[:auto_delete]
+    cmd += ["arguments=#{arguments.to_json}", '-c', '/etc/rabbitmq/rabbitmqadmin.conf']
+    rabbitmqadmin(*cmd)
     @property_hash[:ensure] = :present
   end