include stage reference in body mapping template

docs/guide/overview-of-event-sources.md

@@ -109,6 +109,7 @@ in the `event` object:
 - body
 - method
 - principalId
+- stage
 - headers
 - query
 - path

lib/plugins/aws/deploy/compile/events/apiGateway/README.md

@@ -18,7 +18,7 @@ Those resources are then merged into the `serverless.service.resources.Resources
 
 ## Universal JSON request template
 
-The API Gateway plugin implements a request template which provides `{body, method, principalId, headers, query, path, identity,
+The API Gateway plugin implements a request template which provides `{body, method, principalId, stage, headers, query, path, identity,
 stageVariables} = event` as JavaScript objects. This way you don't have to define the template on your own but can use
 this default template to access the necessary variables in your code.
 

lib/plugins/aws/deploy/compile/events/apiGateway/lib/methods.js

@@ -86,7 +86,15 @@ module.exports = {
 
           // universal velocity template
           // provides
-          // `{body, method, principalId, headers, query, path, identity, stageVariables} = event`
+          // `{ body,
+          //    method,
+          //    principalId,
+          //    stage,
+          //    headers,
+          //    query,
+          //    path,
+          //    identity,
+          //    stageVariables} = event`
           // as js objects
           const DEFAULT_JSON_REQUEST_TEMPLATE = `
             #define( $loop )
@@ -102,6 +110,7 @@ module.exports = {
               "body": $input.json("$"),
               "method": "$context.httpMethod",
               "principalId": "$context.authorizer.principalId",
+              "stage": "$context.stage",
 
               #set( $map = $input.params().header )
               "headers": $loop,