[3.x] Fixes updating site aliases

[nunomaduro]

This pull request fixes updating site aliases on Forge SDK, where using an empty list of aliases ['aliases' => []] as simply being not sent by guzzle.

// Before:
// PUT [...] aliases not updated OK
// PUT ['aliases' => []] aliases not updated KO <--- issue here...
// PUT ['aliases' => ['foo.com']] aliases updated OK

// After:
// PUT [...] aliases not updated OK
// PUT ['aliases' => []] aliases updated to empty OK <--- fixed now...
// PUT ['aliases' => ['foo.com']] aliases updated OK

At this time, because we are not sure of the ramifications of changing form_params by json globally - on the entire SDK - so we decided to apply it on this method (updateSite) only for now.