If user set custom proxy, update enigmail's value too.

1. Install TorBirdy for the first time.
2. Addons - Options - Select "Use custom proxy settings".
3. Change 9150 to 9050. (9150 was the default of this addon)
4. Somehow, TorBirdy automatically changed Enigmail's value.

Expected behavior: ...no-try-dns-srv,http-proxy=socks5h://127.0.0.1:9050 ...

Actual behavior: ...no-try-dns-srv,http-proxy=socks5h://127.0.0.1:9150 ...