Fix deserialization of lambda response

com.fasterxml.jackson.databind.ObjectMapper._convert(ObjectMapper.java:3733)\n\t... 106 more","exception_class":"java.lang.IllegalArgumentException","exception_message":"Cannot deserialize instance of `com.fasterxml.jackson.databind.node.ObjectNode` out of VALUE_STRING token\n at [Source: UNKNOWN; line: -1, column: -1]"},"source_host":"nuxeo-front-62-b6f9k","method":"toResponse","level":"ERROR","message":"java.lang.IllegalArgumentException: Cannot deserialize instance of `com.fasterxml.jackson.databind.node.ObjectNode` out of VALUE_STRING token\n at [Source: UNKNOWN; line: -1, column: -1]","mdc":{"tx":"d200000047544d4944","ServletPath":"\/site","PathInfo":"\/lambda\/success\/1e9e779b-4e35-44dc-a03d-a378e6bd35df","RequestURL":{},"RemoteAddr":"172.16.22.1"},"@timestamp":"2018-08-10T18:41:36.576Z","file":"Web
EngineExceptionMapper.java","line_number":"81","thread_name":"http-nio-0.0.0.0-8080-exec-1","@version":1,"logger_name":"org.nuxeo.ecm.webengine.app.WebEngineExceptionMapper","class<span class="code-quote">":"org.nuxeo.ecm.webengine.app.WebEngineExceptionMapper"}