Fixes #1541: HTTPFilesystem has a race condition on data size between the open and read class, if content changes at server between the 2 class

[masariello]

[martindurant]

I'm pretty sure this will either convert regular HTTPFiles to HTTPStreamFiles (no random access) or fail on seek(.., 2). We should have explicit checks on those conditions to be sure.

[martindurant]

Indeed, this breaks a bunch of tests, like:

>       if start >= self.size or start >= end:
E       TypeError: '>=' not supported between instances of 'int' and 'NoneType'