add fields key to post , put and patch (#3)

* add fields key to post , put and  patch

we can now create update a record with sending array of fields and you can create empty records as api documentation.
```
     \Airtable::table('default')->create( ['name' => 'mohamed']);
     // or empty record
     \Airtable::table('default')->create(null);
   // or with stdclass object
   \Airtable::table('users')->create($user);
```

* Resolve style changes.

Author: mo7amed-3bdalla7

src/Api/AirtableApiClient.php

@@ -60,7 +60,7 @@ public function post($contents = null)
     {
         $url = $this->getEndpointUrl();
 
-        $params = $contents === null ? ['body' => ''] : ['json' => $contents];
+        $params = ['json' => ['fields' => (object) $contents]];
 
         return $this->jsonToObject($this->client->post($url, $params));
     }
@@ -69,7 +69,7 @@ public function put(string $id, $contents = null)
     {
         $url = $this->getEndpointUrl($id);
 
-        $params = $contents === null ? ['body' => ''] : ['json' => $contents];
+        $params = ['json' => ['fields' => (object) $contents]];
 
         return $this->jsonToObject($this->client->put($url, $params));
     }
@@ -78,7 +78,7 @@ public function patch(string $id, $contents = null)
     {
         $url = $this->getEndpointUrl($id);
 
-        $params = $contents === null ? ['body' => ''] : ['json' => $contents];
+        $params = ['json' => ['fields' => (object) $contents]];
 
         return $this->jsonToObject($this->client->patch($url, $params));
     }