Fixed bug when tries to get the resource with secret.

It was unavailable to do next

```
request = Transloadit::Request.new '/assemblies', transloadit.secret
request.get page: 1, auth: transloadit.to_hash
```
and recieve next exception:

```
Transloadit::Request::when performing a GET::with secret#test_0002_must perform a GET against the resource:
TypeError: can't convert Hash into String
    /Users/miry/dev/ruby-sdk/lib/transloadit/request.rb:215:in `hexdigest'
    /Users/miry/dev/ruby-sdk/lib/transloadit/request.rb:215:in `_hmac'
    /Users/miry/dev/ruby-sdk/lib/transloadit/request.rb:202:in `signature'
    /Users/miry/dev/ruby-sdk/lib/transloadit/request.rb:177:in `to_query'
```