Patch if null statement

The new Twitter API (v1.1) will include re-tweets by default. (include_rts)
`$includeRts != null` does not accept false, since that is the same as null in this case.
I've changed all if statements to make sure it also checks the type.

```php
$includeRts = false;
if ($includeRts != null) {
     $parameters['include_rts'] = ($includeRts) ? 'true' : 'false';
}
// $parameters['include_rts'] is not set and since this is true by default in v1.1 there is no way to exclude these

$includeRts = true;
if ($includeRts != null) {
     $parameters['include_rts'] = ($includeRts) ? 'true' : 'false';
}
// $parameters['include_rts'] is equal to 'true'

$includeRts = false;
if ($includeRts !== null) {
     $parameters['include_rts'] = ($includeRts) ? 'true' : 'false';
}
// $parameters['include_rts'] is equal to 'false'
```