Problem with nested array as POST fields.

[stil]

Consider example:

$request=$this->post('http://example.com');
$request->addPostFields(array(
'a'=>array('b'=>'c')
));```
Result:
Notice: Undefined offset: 0 in D:\library\Guzzle\Http\Message\EntityEnclosingRequest.php on line 130

You need to use "hack":
```<?php
$request=$this->post('http://example.com');
$request->setBody(http_build_query(array(
'a'=>array('b'=>'c')
)));```

Why you didn't use [http_build_query](http://php.net/manual/en/function.http-build-query.php) in Guzzle\Http\QueryString to generate query?