为什么nginx重写从/login/ /login发布请求？

有一个if语句，当客户端是Android的时候会重写url。 一切还好。
但是，有些奇怪的东西。
即使if语句的块是bank，Nginx也会将post请求/login写入//login。
所以我有一个404页面。 由于docker服务器只接受/login请求。
服务器conf：

location / { proxy_pass http://localhost:8785/; proxy_set_header Host $http_host; proxy_set_header Remote-Addr $http_remote_addr; proxy_set_header X-Real-IP $remote_addr; if ( $http_user_agent ~ Android ){ # rewrite something, been commented } } 

debugging信息，原始日志https://gist.github.com/3799021

 ... 2012/09/28 16:29:49 [debug] 26416#0: *1 http script regex: "Android" 2012/09/28 16:29:49 [notice] 26416#0: *1 "Android" matches "Android/1.0", client: 106.187.97.22, server: ireedr.com, request: "POST /login HTTP/1.1", host: "ireedr.com" ... 2012/09/28 16:29:49 [debug] 26416#0: *1 http proxy header: "POST //login HTTP/1.0 Host: ireedr.com X-Real-IP: 106.187.97.22 Connection: close Accept-Encoding: identity, deflate, compress, gzip Accept: */* User-Agent: Android/1.0 " ... 2012/09/28 16:29:49 [debug] 26416#0: *1 HTTP/1.1 404 Not Found Server: nginx/1.2.1 Date: Fri, 28 Sep 2012 08:29:49 GMT Content-Type: text/html;charset=ISO-8859-1 Transfer-Encoding: chunked Connection: keep-alive Cache-Control: must-revalidate,no-cache,no-store Content-Encoding: gzip ... 

只有当我在configration文件中注释该块时：

 location / { proxy_pass http://localhost:8785/; proxy_set_header Host $http_host; proxy_set_header Remote-Addr $http_remote_addr; proxy_set_header X-Real-IP $remote_addr; #if ( $http_user_agent ~ Android ){ # #} } 

客户可以得到一个200响应。
debugging信息，原始日志https://gist.github.com/3799023

• 用nginx把基址重写为https
• nginx：URL重写和性能
• 在IIS中向本地TomCat反向代理
• 为什么nginx改变重写规则的HTTP方法？
• 如何在redirect上模拟Apache 标志？

 ... "POST /login HTTP/1.0 Host: ireedr.com X-Real-IP: 106.187.97.22 Connection: close Accept-Encoding: identity, deflate, compress, gzip Accept: */* User-Agent: Android/1.0 " ... 2012/09/28 16:27:19 [debug] 26319#0: *1 HTTP/1.1 200 OK Server: nginx/1.2.1 Date: Fri, 28 Sep 2012 08:27:19 GMT Content-Type: application/json;charset=UTF-8 Content-Length: 17 Connection: keep-alive ... 

作为日志：

 2012/09/28 16:29:49 [notice] 26416#0: *1 "Android" matches "Android/1.0", client: 106.187.97.22, server: ireedr.com, request: "POST /login HTTP/1.1", host: "ireedr.com" 2012/09/28 16:29:49 [debug] 26416#0: *1 http script if 2012/09/28 16:29:49 [debug] 26416#0: *1 post rewrite phase: 4 2012/09/28 16:29:49 [debug] 26416#0: *1 generic phase: 5 2012/09/28 16:29:49 [debug] 26416#0: *1 generic phase: 6 2012/09/28 16:29:49 [debug] 26416#0: *1 generic phase: 7 2012/09/28 16:29:49 [debug] 26416#0: *1 access phase: 8 2012/09/28 16:29:49 [debug] 26416#0: *1 access phase: 9 2012/09/28 16:29:49 [debug] 26416#0: *1 access phase: 10 2012/09/28 16:29:49 [debug] 26416#0: *1 post access phase: 11 2012/09/28 16:29:49 [debug] 26416#0: *1 try files phase: 12 2012/09/28 16:29:49 [debug] 26416#0: *1 posix_memalign: 0000000001E798F0:4096 @16 2012/09/28 16:29:49 [debug] 26416#0: *1 http init upstream, client timer: 0 2012/09/28 16:29:49 [debug] 26416#0: *1 epoll add event: fd:13 op:3 ev:80000005 2012/09/28 16:29:49 [debug] 26416#0: *1 http script copy: "Host: " 2012/09/28 16:29:49 [debug] 26416#0: *1 http script var: "ireedr.com" 2012/09/28 16:29:49 [debug] 26416#0: *1 http script copy: " " 2012/09/28 16:29:49 [debug] 26416#0: *1 http script copy: "" 2012/09/28 16:29:49 [debug] 26416#0: *1 http script copy: "" 2012/09/28 16:29:49 [debug] 26416#0: *1 http script copy: "X-Real-IP: " 2012/09/28 16:29:49 [debug] 26416#0: *1 http script var: "106.187.97.22" 2012/09/28 16:29:49 [debug] 26416#0: *1 http script copy: " " 2012/09/28 16:29:49 [debug] 26416#0: *1 http script copy: "Connection: close " 2012/09/28 16:29:49 [debug] 26416#0: *1 http proxy header: "Accept-Encoding: identity, deflate, compress, gzip" 2012/09/28 16:29:49 [debug] 26416#0: *1 http proxy header: "Accept: */*" 2012/09/28 16:29:49 [debug] 26416#0: *1 http proxy header: "User-Agent: Android/1.0" 2012/09/28 16:29:49 [debug] 26416#0: *1 http proxy header: "POST //login HTTP/1.0 Host: ireedr.com X-Real-IP: 106.187.97.22 Connection: close Accept-Encoding: identity, deflate, compress, gzip Accept: */* User-Agent: Android/1.0 " ... 

也许后重写阶段已经重写了请求。
有人可以帮我解决这个问题，或知道为什么nginx做到这一点？
非常感激。