can't conect after 5.1.8 using xchacha20-ietf-poly1305

[hellojack312]

Please read contributing guidelines. Thanks.

can't connect after version5.1.7 using xchacha20-ietf-poly1305 method
version 5.1.7 work perfectly.

Steps to reproduce the behavior:

1. xiaomi4 miui 10.1 android version6.0.1
   2.method xchacha20-ietf-poly1305

Expected behavior
.after update shadowsocks-rust to 1.9.0 (in version 5.1.8 #2622)this problem occur.
hope can fix this problem.don't know how to upload screenshots.just can't connect
Screenshots
If applicable, add screenshots to help explain your problem.

Smartphone (please complete the following information):

• Android 6.0.1
• Device: xiaomi4
• Version: miui10.1
• Last version that did not exhibit the issue: [not applicable]

Configuration
Put an x inside the [ ] that applies.
all by default

• [x ] IPv4 server address
• IPv6 server address
• [x ] Client IPv4 availability
• Client IPv6 availability
• Encrypt method:xchacha20-ietf-poly1305
• Route
  • [x ] All
  • Bypass LAN
  • Bypass China
  • Bypass LAN & China
  • GFW List
  • China List
  • Custom rules
• IPv6 route
• Apps VPN mode
  • Bypass mode
• Remote DNS: 8.8.8.8
• DNS over UDP
• Plugin configuration (if applicable):
• Auto Connect
• TCP Fast Open
• If you're not using VPN mode, please supply more details here:

Additional context
Add any other context about the problem here.

[dev4u]

使用chacha20-ietf-poly1305替换。