I have a @HttpPost method in @RestResource class:

@RestResource(urlMapping='/Something/*') global with sharing class HttpClass { @HttpPost global static String updateCustomObject() { //Fetching request RestRequest req = RestContext.request; //Getting JSON String from request body String jsonInput = req.requestBody.toString(); JSONParser parser = JSON.createParser(jsonInput); System.debug('Parser:' + jsonInput); jsonInput = jsonInput.replace(' ',''); Object jsonUnpacked = json.deserializeUntyped(jsonInput); System.debug(jsonUnpacked); User gUser = (User)JSON.deserialize(jsonInput, User.class); system.debug('user ' + gUser); X2017_questionnaire__c quest = new X2017_questionnaire__c(Name = gUser.name,Email_address__c = gUser.email,Pain_Factor_1__c = gUser.pain1, Pain_Factor_2__c = gUser.pain2,Pain_Factor_3__c = gUser.pain3, Pain_Factor_4__c = gUser.pain4,Pain_Factor_5__c = gUser.pain5, Project_Objective_1__c = gUser.obj1,Project_Objective_2__c = gUser.obj2, Project_Objective_3__c = gUser.obj3,Project_Objective_4__c = gUser.obj4, Project_Objective_5__c = gUser.obj5); insert quest; /* //Below JSON Generator create response string JSONGenerator jsonGenerator = JSON.createGenerator(true); jsonGenerator.writeStartArray(); jsonGenerator.writeStartObject(); //jsonGenerator.writeStringField('Email',emailFromJSONInput); jsonGenerator.writeStringField('Status', 'Success/Fail'); jsonGenerator.writeEndObject(); jsonGenerator.writeEndArray(); String responseJSONString = jsonGenerator.getAsString();*/ List<X2017_questionnaire__c> lstOfQuest = [SELECT name,Email_address__c FROM X2017_questionnaire__c WHERE Email_address__c =:gUser.email]; if (!lstOfQuest.isEmpty()){ return 'Created Succesfully'; }else { return 'Error'; } } public class User { public String name; public String email; public String pain1; public String pain2; public String pain3; public String pain4; public String pain5; public String obj1; public String obj2; public String obj3; public String obj4; public String obj5; } } 

How do I write a test class for the HTTPPost method? I tried using HttpMockCallout, but it works with HttpResponse and for my @HttpPost method I need a HttpRequest test.