Include request body of multipart/form-data requests into AWS4 signature

lib/authorizer/aws4.js

@@ -71,9 +71,16 @@
             return callback(hash.digest(digestEncoding));
         }
 
-        // @todo: formdata body type requires adding new data to form instead of setting headers for AWS auth.
+        // @todo: formdata body type for S3 requires adding new data to form instead of setting headers for AWS auth.
         //        Figure out how to do that. See below link:
         //        AWS auth with formdata: https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-post-example.html
+        //        For now, sign requests as other POST requests for use with API gateway and Lambda functions.
+        if (body.mode === RequestBody.MODES.formdata) {
+            formdataBody = bodyBuilder.formdata(body.formdata).body;
+            hash.update(formdataBody);
+
+            return callback(hash.digest(digestEncoding));
+        }
 
         // ensure that callback is called if body.mode doesn't match with any of the above modes
         return callback();