Fixes to make SO login work. GitHub now works also (was a FF plugin bug)

sideload/socialnorm/stackoverflow/src/StackOverflowProvider.php

@@ -1,5 +1,7 @@
 <?php namespace SocialNorm\StackOverflow;
 
+use SocialNorm\Exceptions\AuthNotUsableException;
+use SocialNorm\Exceptions\InvalidAuthorizationCodeException;
 use SocialNorm\Providers\OAuth2Provider;
 use GuzzleHttp\Client as HttpClient;
 use SocialNorm\Request;
@@ -21,6 +23,7 @@ class StackOverflowProvider extends OAuth2Provider
     protected $headers = [
         'authorize' => [],
         'access_token' => [
+            'Accept' => 'application/json',
             'Content-Type' => 'application/x-www-form-urlencoded'
         ],
         'user_details' => [],
@@ -65,12 +68,21 @@ class StackOverflowProvider extends OAuth2Provider
 
     protected function parseTokenResponse($response)
     {
-        return $this->parseJsonTokenResponse($response);
+        return explode('=', $response, 2)[1];
     }
 
     protected function parseUserDataResponse($response)
     {
-        return json_decode($response, true);
+        $decoded = (array)json_decode($response, true);
+
+        if ($decoded === false || json_last_error())
+            throw new InvalidAuthorizationCodeException('Corrupt response json: ' . $response . ", error " . json_last_error_msg());
+
+        if (0 == count(array_get($decoded, 'items'))) {
+            throw new AuthNotUsableException('No profile on StackOverflow. Resp: '.$response);
+        }
+
+        return $decoded;
     }
 
     protected function userId()