Upgrading Servo to 0.9.2 causes a panic to occur when using connection pooling

[jdm]

No debug build handy, but I got this when upgrading from 0.9.1 to 0.9.2:

thread 'http_loader for https://www.redditstatic.com/subscribe-header.svg' panicked at 'called `Option::unwrap()` on a `None` value', ../src/libcore/option.rs:325
stack backtrace:
   1:        0x10de64d68 - std::sys::backtrace::tracing::imp::write::h9fb600083204ae7f
   2:        0x10de6b065 - std::panicking::default_hook::_$u7b$$u7b$closure$u7d$$u7d$::hca543c34f11229ac
   3:        0x10de6ac7e - std::panicking::default_hook::hc2c969e7453d080c
   4:        0x10d73bdc2 - util::panicking::initiate_panic_hook::_$u7b$$u7b$closure$u7d$$u7d$::_$u7b$$u7b$closure$u7d$$u7d$::he920cf7dc1208a16
   5:        0x10de52618 - std::sys_common::unwind::begin_unwind_inner::h30e12d15ce2b2e25
   6:        0x10de53f4e - std::sys_common::unwind::begin_unwind_fmt::hb2de8a9968d38523
   7:        0x10de64037 - rust_begin_unwind
   8:        0x10de92e20 - core::panicking::panic_fmt::h257ceb0aa351d801
   9:        0x10de9311c - core::panicking::panic::h4bb1497076d04ab9
  10:        0x10c9ee54d - _<http_loader..NetworkHttpRequestFactory as http_loader..HttpRequestFactory>::create::hb14b293d2f174883
  11:        0x10c9dff55 - net::http_loader::obtain_response::h35f9188afca4a9eb
  12:        0x10c9d65a9 - net::http_loader::load::ha96d58df29b69795
  13:        0x10c9d2bd9 - std::sys_common::unwind::try::try_fn::h68b713da4550c11a
  14:        0x10de63fcb - __rust_try
  15:        0x10de63f53 - std::sys_common::unwind::inner_try::h47a4d9cd4a369dcd
  16:        0x10c9d41ad - _<F as std..boxed..FnBox<A>>::call_box::h83c573ecb2eb7774
  17:        0x10de6a0e8 - std::sys::thread::Thread::new::thread_start::h6f266e069bf4ec2b
  18:     0x7fff96a57898 - _pthread_body
  19:     0x7fff96a57729 - _pthread_start

It was followed by PoisonError panics from other HTTP loader threads, which suggests that the first panic occurred while the pool mutex was locked.