Passing boolean to 'call' causes "Invalid MsgPack - packet length"

Hi!
I have a problem with tarantool 1.6 and php-connector from php7-v1 branch.

How to reproduce:
just pass boolean value to 'call':

```
$tnt = new Tarantool('localhost', 3301);
$tnt->call([true]);
```

You should see this in tarantool log:

```
iproto iproto.cc:450 E> ER_INVALID_MSGPACK: Invalid MsgPack - packet length
```

P.S.
Tarantool version: 1.6.8-653-g2e4a17d
