Rename AbstractApi to HttpApi (#214)

Fix #200

src/Mailgun/Api/Domain.php

@@ -27,7 +27,7 @@ use Psr\Http\Message\ResponseInterface;
  *
  * @author Sean Johnson <sean@mailgun.com>
  */
-class Domain extends AbstractApi
+class Domain extends HttpApi
 {
     /**
      * Returns a list of domains on the account.

src/Mailgun/Api/HttpApi.php

@@ -18,7 +18,7 @@ use Psr\Http\Message\ResponseInterface;
 /**
  * @author Tobias Nyholm <tobias.nyholm@gmail.com>
  */
-abstract class AbstractApi
+abstract class HttpApi
 {
     /**
      * The HTTP client.

src/Mailgun/Api/Stats.php

@@ -11,7 +11,7 @@ use Mailgun\Resource\Api\Stats\TotalResponse;
  *
  * @author Tobias Nyholm <tobias.nyholm@gmail.com>
  */
-class Stats extends AbstractApi
+class Stats extends HttpApi
 {
     /**
      * @param string $domain