stream_get_contents() doesn't read the whole response

### HHVM Version

HipHop VM 3.12.1 (rel)
Compiler: tags/HHVM-3.12.1-0-gf516f1bb9046218f89885a220354c19dda6d8f4d
Repo schema: f2e5f39b2ad4a08bcbd90b5d8bcb580f40fba6c8
### Standalone code, or other way to reproduce the problem

server.php

``` php
$stream = stream_socket_server('0.0.0.0:8888', $errorCode, $errorMessage);

if (!$stream) {
die($errorMessage);
}

$conn = stream_socket_accept($stream);

$len = 1024 * 1024;
$data = str_repeat('x', $len);

fwrite($conn, $data, $len);
fclose($conn);

fclose($stream);
```

client.php

``` php
$stream = stream_socket_client('0.0.0.0:8888', $errorCode, $errorMessage, 3.0);

if (!$stream) {
die($errorMessage);
}

$data = stream_get_contents($stream, 1024 * 1024);

var_dump(strlen($data));
```
### Expected result

``` sh
$ php tests/socket/server.php &
$ php tests/socket/client.php
int(1048576)
[1]+ Done php tests/socket/server.php
```
### Actual result

``` sh
$ hhvm tests/socket/server.php &
$ hhvm tests/socket/client.php
int(8192)
[1]+ Done hhvm tests/socket/server.php
```