Hosting on GH Pages doesn't work after allowing UrlVfs to support downloads without length

I have "unsupported" error logged in console when my game hosted on GitHub pages reads something from `resourcesVfs`: https://servb.github.io/pph/.

The stacktrace is as follows:
```
stt@https://servb.github.io/pph/PPH.js:1:66527
f_t@https://servb.github.io/pph/PPH.js:1:78130
vvt@https://servb.github.io/pph/PPH.js:1:173929
_/Ryt.prototype.doResume_0_k$@https://servb.github.io/pph/PPH.js:1:1163768
_/Iyt.prototype.getLength_0_k$@https://servb.github.io/pph/PPH.js:1:1164457
_/Wgt.prototype.getLength_0_k$@https://servb.github.io/pph/PPH.js:1:1235904
_/Ngt.prototype.getLength_0_k$@https://servb.github.io/pph/PPH.js:1:1230018
_/L$t.prototype.doResume_0_k$@https://servb.github.io/pph/PPH.js:1:813992
_/N$t.prototype.doResume_0_k$@https://servb.github.io/pph/PPH.js:1:813308
_/C$t.prototype.invoke_u23hy3_k$@https://servb.github.io/pph/PPH.js:1:813715
_/Gyt.prototype.doResume_0_k$@https://servb.github.io/pph/PPH.js:1:1179499
_/Byt.prototype.doResume_0_k$@https://servb.github.io/pph/PPH.js:1:1179071
_/zet.prototype.resumeWith_jccoe6_k$@https://servb.github.io/pph/PPH.js:1:572195
_/zet.prototype.resumeWith_bnunh2_k$@https://servb.github.io/pph/PPH.js:1:572511
_/cft.prototype.run_sv8swh_k$@https://servb.github.io/pph/PPH.js:1:770171
_/GYt.prototype.executePending_zah5t7_k$@https://servb.github.io/pph/PPH.js:1:1732452
_/uJt.prototype.invoke_okcceq_k$@https://servb.github.io/pph/PPH.js:1:1759970
hJt/this._jsFrame
```

I've debugged a bit and see that `HEAD https://servb.github.io/pph/resources.zip` request doesn't return Content-Length header, so `UrlVfs$AsyncStreamBase::getLength` returns `unsupported()`:


It seems that GitHub Pages just doesn't provide this header. When launching locally via `runJs`, everything works fine. I guess my game worked on GH Pages before b9cfc00, but now it doesn't.