ClientInterface::request did not accept null
ClientInterface::request did not accept null

I found a difference between the ClientInterface and the Client Implementation Request Method.
The `$uri` is required in the Interface, however in the Implementation it is allowed to be null.
See:
https://github.com/guzzle/guzzle/blob/c6851d6e48f63b69357cbfa55bca116448140e0c/src/Client.php#L126 and https://github.com/guzzle/guzzle/blob/c6851d6e48f63b69357cbfa55bca116448140e0c/src/ClientInterface.php#L54

To fix this difference, i changed the `$uri` Parameter in the Interface to default Value `null`
mortenhauberg/guzzle,wherefortravel/guzzle,thinsoldier/guzzle,dshafik/guzzle,paveljanda/guzzle,BostjanOb/guzzle,ehough/guzzle,kkopachev/guzzle,wherefortravel/guzzle,BostjanOb/guzzle,ehough/guzzle,Brunty/guzzle,guzzle/guzzle,michabbb-backup/guzzle,dshafik/guzzle,Tobion/guzzle,Tobion/guzzle,mortenhauberg/guzzle,Brunty/guzzle,guzzle/guzzle,paveljanda/guzzle,michabbb-backup/guzzle,kkopachev/guzzle,thinsoldier/guzzle