Configurable Headers for All Requests to PayPal

- Allows adding additional headers to every call to PayPal APIs.
- Refactored OAuthTokenCredential to share code for making Rest Calls
- SDK Config to add headers with http.headers.* syntax
- Fixes #193

lib/PayPal/Rest/RestHandler.php

@@ -84,6 +84,11 @@ class RestHandler implements IPPHandler
         if ($httpConfig->getMethod() == 'POST' || $httpConfig->getMethod() == 'PUT') {
             $httpConfig->addHeader('PayPal-Request-Id', $this->apiContext->getRequestId());
         }
+        // Add any additional Headers that they may have provided
+        $headers = $this->apiContext->getRequestHeaders();
+        foreach ($headers as $key => $value) {
+            $httpConfig->addHeader($key, $value);
+        }
     }
 
     /**