Support GET request

It seems that `WebTestClient` doesn't support GET request, so I changed it.

versions:

- php: PHP 5.4.30
- there4/slim-test-helpers: master
- [codeguy/Slim](https://github.com/codeguy/Slim): 2.4.3

I create this test case:

```php
<?php

use Slim\Slim;
use There4\Slim\Test\WebTestCase;

class SampleTest extends WebTestCase
{
    /**
     * @inheritdoc
     */
    public function getSlimInstance()
    {
        $app = new Slim(array(
            'debug' => false,
        ));

        // GET /messages
        $app->get('/messages', function() use ($app) {
            $request = $app->request();
            $user_id = $request->get('user_id');

            echo 'user_id:' . $user_id;
        });

        // POST /messages
        $app->post('/messages', function() use ($app) {
            $request = $app->request();
            $message = $request->post('message');

            echo 'message:' . $message;
        });

        return $app;
    }

    /**
     * @test
     */
    public function testPost()
    {
        $this->client->post('/messages', [
            'message' => 'hogehoge',
        ]);
        $this->assertSame('message:hogehoge', $this->client->response->getBody());
    }

    /**
     * @test
     */
    public function testGet()
    {
        $this->client->get('/messages', [
            'user_id' => 1234,
        ]);
        $this->assertSame('user_id:1234', $this->client->response->getBody());
    }
}
```

And execute this test with phpunit.

#### Test Result

Before:
```
.F

Time: 79 ms, Memory: 6.75Mb

There was 1 failure:

1) SampleTest::testGet
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-user_id:1234
+user_id:
```

After:
```
..

Time: 75 ms, Memory: 6.75Mb

OK (2 tests, 2 assertions)
```