[core] Fix LocalServer failure to handle large body request (Issue awslabs#481) (awslabs#504)

Fix awslabs#481

Author: sebsto

Sources/AWSLambdaRuntime/Lambda+LocalServer.swift

@@ -238,7 +238,7 @@ private struct LambdaHTTPServer {
                         requestHead = head
 
                     case .body(let body):
-                        requestBody = body
+                        requestBody.setOrWriteImmutableBuffer(body)
 
                     case .end:
                         precondition(requestHead != nil, "Received .end without .head")