guest trap with `stream or future write canceled when no write is pending`

[rvolosatovs]

On dabcd77:

cargo test -p wasmtime-wasi-http 'p3::wasi_http_proxy_tests'

traps in the guest with:

test p3::wasi_http_proxy_tests ... FAILED

failures:

---- p3::wasi_http_proxy_tests stdout ----
Error: error while executing at wasm backtrace:
    0: 0x10097 - api_0_3_proxy-a77f16bec6abd300.wasm!_ZN14wit_bindgen_rt13async_support14future_support24CancelableWrite$LT$T$GT$10cancel_mut28_$u7b$$u7b$closure$u7d$$u7d$17h7ae0d582d61ea87bE
    1: 0x3007 - api_0_3_proxy-a77f16bec6abd300.wasm!_ZN14wit_bindgen_rt13async_support10with_entry17hf71adfa8babb451dE
    2: 0xfed1 - api_0_3_proxy-a77f16bec6abd300.wasm!_ZN14wit_bindgen_rt13async_support14future_support24CancelableWrite$LT$T$GT$10cancel_mut17h23d7ba11d1b48247E
    3: 0x10f8d - api_0_3_proxy-a77f16bec6abd300.wasm!_ZN113_$LT$wit_bindgen_rt..async_support..future_support..CancelableWrite$LT$T$GT$$u20$as$u20$core..ops..drop..Drop$GT$4drop17hd95ed187f1b34333E
    4: 0x124b0 - api_0_3_proxy-a77f16bec6abd300.wasm!_ZN4core3ptr241drop_in_place$LT$wit_bindgen_rt..async_support..future_support..CancelableWrite$LT$core..result..Result$LT$core..option..Option$LT$test_programs..p3..wasi..http..types..Fields$GT$$C$test_programs..p3..wasi..http..types..ErrorCode$GT$$GT$$GT$17h84983ef7394c926eE
    5: 0x83d9 - api_0_3_proxy-a77f16bec6abd300.wasm!_ZN98_$LT$api_0_3_proxy..T$u20$as$u20$test_programs..p3..proxy..exports..wasi..http..handler..Guest$GT$6handle28_$u7b$$u7b$closure$u7d$$u7d$28_$u7b$$u7b$closure$u7d$$u7d$17h3929da2a2dc563e3E
    6: 0x458c5 - api_0_3_proxy-a77f16bec6abd300.wasm!_ZN72_$LT$core..pin..Pin$LT$P$GT$$u20$as$u20$core..future..future..Future$GT$4poll17h26bb2cf98b8ed1d6E
    7: 0x47a0b - api_0_3_proxy-a77f16bec6abd300.wasm!_ZN117_$LT$futures_util..stream..futures_unordered..FuturesUnordered$LT$Fut$GT$$u20$as$u20$futures_core..stream..Stream$GT$9poll_next17h8ca91f12fbdc78e3E
    8: 0x48d94 - api_0_3_proxy-a77f16bec6abd300.wasm!_ZN12futures_util6stream6stream9StreamExt15poll_next_unpin17hd3a40fbf482ceb17E
    9: 0x4b207 - api_0_3_proxy-a77f16bec6abd300.wasm!_ZN14wit_bindgen_rt13async_support4poll17h9789b40aa88ec685E
   10: 0x4bbc4 - api_0_3_proxy-a77f16bec6abd300.wasm!_ZN14wit_bindgen_rt13async_support19callback_with_state17h47ce49e4742f6fccE
   11: 0x4b922 - api_0_3_proxy-a77f16bec6abd300.wasm!_ZN14wit_bindgen_rt13async_support8callback17hab0017782fa3345fE
   12: 0x1643e - api_0_3_proxy-a77f16bec6abd300.wasm!_ZN13test_programs2p35proxy7exports4wasi4http7handler17__callback_handle17h9544517f67c209b0E
   13: 0x147f4 - api_0_3_proxy-a77f16bec6abd300.wasm![callback][async-lift]wasi:http/handler@0.3.0-draft#handle

Caused by:
    stream or future write canceled when no write is pending

[rvolosatovs]

Looks like this was fixed by #94 and/or test refactoring. It appears that this was a symptom of a future not fully awaited by the guest