Added the awaitInitialization() call as suggested in #71.

Author: sapessi

aws-serverless-java-container-spark/src/main/java/com/amazonaws/serverless/proxy/spark/SparkLambdaContainerHandler.java

@@ -170,6 +170,10 @@ protected void handleRequest(AwsProxyHttpServletRequest httpServletRequest, AwsH
             // manually add the spark filter to the chain. This should the last one and match all uris
             FilterRegistration.Dynamic sparkRegistration = getServletContext().addFilter("SparkFilter", embeddedServer.getSparkFilter());
             sparkRegistration.addMappingForUrlPatterns(EnumSet.of(DispatcherType.REQUEST), true, "/*");
+
+            // adding this call to make sure that the framework is fully initialized. This should address a race
+            // condition and solve GitHub issue #71.
+            Spark.awaitInitialization();
         }
 
         doFilter(httpServletRequest, httpServletResponse, null);