Update Unirest.php

Void notice when given querystring params miss the '=' equal. 
For example, in the following example, the call would print a Notice : 
```
Unirest::get('https://www.googleapis.com/freebase/v1/mqlread?query=[{"type":"/music/album","name":null,"artist":{"id":"/en/bob_dylan"},"limit":3}]&cursor');
```
as you see the last keyword curson has no '=' sign
```
Notice: Undefined offset: 1 in ..... Unirest/Unirest.php on line 239
```

The url  used in the example was took from a Google tutorial
https://developers.google.com/freebase/v1/mql-overview#querying-with-cursor-paging-results