zero-length reads break new mutable downloader

[warner]

The new mutable downloader, landed as part of MDMF in #393, has problems when asked to download zero bytes: it returns more than zero bytes.

We have tests in test_mutable.Version to catch this, currently disabled.

This is a blocker for 1.9beta

[kevan]

fix zero-length reads; enable tests of zero-length reads

[Kevan Carstensen <kevan@…>]

In 32e30c9023479e3c:

mutable/retrieve: handle the case where self._read_length is 0.

Note that the downloader will still fetch a segment for a zero-length
read, which is wasteful. Fixing that isn't specifically required to fix
#1512, but it should probably be fixed before 1.9.

[warner]

I agree with the comment that it'd be nice to improve efficiency of zero-byte reads by not actually downloading any segments (I've opened #1518 to track that), but I'm not going to hold up 1.9 for that.. correctness is sufficient.