Fix variable typo

[sammyskills]

• [x] Corrected $this->callbake_url to $this->callback_url.
• [x] Declare vars before usage in method calls.

[sammyskills]

See this: https://github.com/datamweb/shield-oauth/blob/4da6cdfd7aec4590b0bf9d7e6c880b6508f82763/src/Libraries/GoogleOAuth.php#L62

What class of Exception will be called here?

Asking because my text editor is highlighting it as an error: Use of unknown class: 'Datamweb\ShieldOAuth\Libraries\Exception

[datamweb]

What class of Exception will be called here?

I intended to fix these problems in #19 , but I lost the energy to do so. Maybe this is a good warning for me to continue.

You can solve the problem by adding use Exception;.

[sammyskills]

You can solve the problem by adding use Exception;.

Added to commit.