don't get resource length from Content-Length of 206 Partial Content response with no complete-length
and use resource lengths available even when not the initial request.
This has the effect of using the most recent indication of resource length.
Differential Revision: https://phabricator.services.mozilla.com/D146276
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1765025
gecko-commit: 59d1800b85db220f0d951583e89bb2f9924a11e6
gecko-reviewers: padenot