SparkJava request.body() consumes payload in first defined before() filter

*To help us debug your issue fill in the basic information below using the options provided*

*Serverless Java Container version*: `1.5.1`

*Implementations:* `Spark`

*Framework version:* `2.9.2`

*Frontend service:* `REST API`

*Deployment method:* `Lambda, Request is sent with AwsProxyRequest`

## Scenario
Previously we were using combination : SparkJava 2.9.2 + AwsServerless 1.3.1 and everything was working fine.
Just few days ago we tried to upgrade AwsServerless to 1.5.1 and noticed that POST works incorrectly.

## Expected behavior
request.body() should be available for getting any number of times in filters, handlers.
It was working fine in 1.3.1

## Actual behavior
request.body() returns value only in first specified filter. Then it returns "" (empty String)
It stopped working after we updated to 1.5.1

## Steps to reproduce
To reproduce the issue, simply run RequestStreamLambdaHandler.main() with no any prerequisites.
Just check the version in build.gradle before debug.
```
apply plugin: 'java'
apply plugin: 'idea'

repositories {
mavenCentral()
}

dependencies {
compile 'com.sparkjava:spark-core:2.9.2'
//compile 'com.amazonaws.serverless:aws-serverless-java-container-spark:1.3.1'
compile 'com.amazonaws.serverless:aws-serverless-java-container-spark:1.5.1'
}

```

RequestStreamLambdaHandler.java
```
package hello;

import com.amazonaws.serverless.exceptions.ContainerInitializationException;
import com.amazonaws.serverless.proxy.model.AwsProxyRequest;
import com.amazonaws.serverless.proxy.model.AwsProxyResponse;
import com.amazonaws.serverless.proxy.spark.SparkLambdaContainerHandler;
import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestStreamHandler;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

import spark.Request;
import spark.Response;
import spark.Spark;

import static spark.Spark.before;
import static spark.Spark.post;

public class RequestStreamLambdaHandler implements RequestStreamHandler {
@Override
public void handleRequest(InputStream inputStream, OutputStream outputStream, Context context) throws IOException {
SparkLambdaContainerHandler handler;
try {
handler = SparkLambdaContainerHandler.getAwsProxyHandler();
defineBaseResources();
Spark.awaitInitialization();
} catch (ContainerInitializationException e) {
return;
}

handler.proxyStream(inputStream, outputStream, context);
}

public void defineBaseResources() {
// define base routes
before("/*", this::beforeFilter1);
before("/*", this::beforeFilter2);
before("/*", this::beforeFilter3);

post("/post-request", this::handlePost);
}

private Request handlePost(Request req, Response res) {
System.out.println("POST Handler Request Body: " + req.body());
return req;
}

protected void beforeFilter1(Request req, Response res) {
System.out.println("Filter 1 Request Body: " + req.body());
}

protected void beforeFilter2(Request req, Response res) {
System.out.println("Filter 2 Request Body: " + req.body());
}

protected void beforeFilter3(Request req, Response res) {
System.out.println("Filter 3 Request Body: " + req.body());
}

public static void main(String[] args) throws Exception {
String body = "{\"path\": \"/post-request\","
+ " \"httpMethod\": \"POST\","
+ " \"requestContext\": {"
+ " \"identity\": {}"
+ " },"
+ " \"body\":\"{\\\"field1\\\":\\\"value1\\\"}\""
+ "}";


RequestStreamLambdaHandler handler = new RequestStreamLambdaHandler();
final ByteArrayOutputStream baos = new ByteArrayOutputStream();
handler.handleRequest(new ByteArrayInputStream(body.getBytes()), baos, null);
}
}
```

## Full log output
*Paste the full log output from the Lambda function's CloudWatch logs*

Log while 1.3.1 is used :
```
> Task :RequestStreamLambdaHandler.main()
Filter 1 Request Body: {"field1":"value1"}
Filter 2 Request Body: {"field1":"value1"}
Filter 3 Request Body: {"field1":"value1"}
POST Handler Request Body: {"field1":"value1"}
```

Log while 1.5.1 is used :
```
> Task :RequestStreamLambdaHandler.main()
Filter 1 Request Body: {"field1":"value1"}
Filter 2 Request Body:
Filter 3 Request Body:
POST Handler Request Body:
```

Start with build.gradle and RequestStreamLambdaHandler.java, then run its main method with the Spark container versions 1.3.1 and 1.5.1 to reproduce the differing logs. Trace SparkLambdaContainerHandler.proxyStream and the request-body handling, and consider the issue done when request.body() returns the payload in every before filter and the handler.

索引モデルが issue の本文から書いたものです。