修正验签时间戳HTTP头拼写, fixes #10

Author: xy-peng

src/main/java/com/wechat/pay/contrib/apache/httpclient/auth/WechatPay2Validator.java

@@ -36,7 +36,7 @@ public final boolean validate(CloseableHttpResponse response) throws IOException
   }
 
   protected final String buildMessage(CloseableHttpResponse response) throws IOException {
-    String timestamp = response.getFirstHeader("Wechatpay-TimeStamp").getValue();
+    String timestamp = response.getFirstHeader("Wechatpay-Timestamp").getValue();
     String nonce = response.getFirstHeader("Wechatpay-Nonce").getValue();
 
     String body = getResponseBody(response);