bug#19439: guix download fails with "bad qstring header component"

[Eric Bavier <ericbavier@gmail.com>]

I've encountered the same issue while attempting to upgrade the
python-pillow package.  For the newer versions 2.6.2 and 2.7.0, the
following errors are reported when downloading the source::

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
starting download of
`/gnu/store/i40q1mk9dnkg2mfyvfhjyq8lcywmgvh1-Pillow-2.7.0.tar.gz' from
`https://pypi.python.org/packages/source/P/Pillow/Pillow-2.7.0.tar.gz'...
ERROR: Bad qstring header component: 3fdc9f16a1b60366ae8158b424655d10

failed to download
"/gnu/store/i40q1mk9dnkg2mfyvfhjyq8lcywmgvh1-Pillow-2.7.0.tar.gz" from
"https://pypi.python.org/packages/source/P/Pillow/Pillow-2.7.0.tar.gz"
builder for
`/gnu/store/bij5w1kppj9wiqgarmsa3nc8z73i1v6l-Pillow-2.7.0.tar.gz.drv'
failed to produce output path
`/gnu/store/i40q1mk9dnkg2mfyvfhjyq8lcywmgvh1-Pillow-2.7.0.tar.gz'
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I don't have any ideas on how to work around this issue in general.  In
this case, we might be able to fetch the source from github rather than
pypi.  But this should add to the question of how frequent this problem
is.

-- 
Eric Bavier