remove overuse of the toString() and correct variable naming convention

Author: BL

lib/AmazonMwsResource.js

@@ -255,8 +255,8 @@ AmazonMwsResource.prototype = {
                         /**
                          * https://github.com/ashtuchkin/iconv-lite/issues/32
                          */
-                        var str = iconv.decode(bufferString, 'win1251').toString();
-                        responseString = iconv.encode(str, charset).toString();
+                        var win1251String = iconv.decode(bufferString, 'win1251');
+                        responseString = iconv.encode(win1251String, charset);
                     } catch (Exception) {
                         debug('Exception iconv ', Exception);
                         return callback.call(self, new Error.AmazonMwsAPIError({