improve Batch feature

@lestrrat 

Hi, 

I modified some codes because I guess the implementation of the batch is not as per spec. It should return a response as an array when it receives a request as an array.
https://github.com/ar-tama/JSON-RPC/commit/56df2fef1189ab52216861b910c76bcd033d89ec#diff-71239e424b2f3ec2355130ff1a5ddbf6R237 is a test case (corresponding to the spec).

before:
```
--> [ {"jsonrpc": "2.0", "method": "subtract", "params": [42, 23], "id": 1} ] # batch
<-- {"jsonrpc": "2.0", "result": 19, "id": 1} # not batch
```
after:
```
--> [ {"jsonrpc": "2.0", "method": "subtract", "params": [42, 23], "id": 1} ] # batch
<-- [ {"jsonrpc": "2.0", "result": 19, "id": 1} ] # batch
```