downloader: duplicate downloads in parallel case

[tikkss]

Steps to reproduce

Clear the cache for example AFINN and then runs the tests in thread-parallel mode:

$ ruby -e 'require "datasets"; Datasets::AFINN.new.clear_cache!' && ruby test/run.rb -t AFINNTest --parallel=thread

Expected result

Only one download of imm6010.zip happens during the test run:

Loaded suite test
Started
imm6010.zip - 100.0% [ 16.23KB/ 16.23KB] 00:00:00   7KB/s
Finished in 9.103834 seconds.
---------------------------------------------------------------------------------
2 tests, 2 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed
---------------------------------------------------------------------------------
0.22 tests/s, 0.22 assertions/s

Actual result

The same file imm6010.zip is downloaded twice:

Loaded suite test
Started
imm6010.zip - 100.0% [ 16.23KB/ 16.23KB] 00:00:00   7KB/s
imm6010.zip - 100.0% [ 16.23KB/ 16.23KB] 00:00:00   7KB/s
Finished in 7.941769 seconds.
---------------------------------------------------------------------------------
2 tests, 2 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed
---------------------------------------------------------------------------------
0.25 tests/s, 0.25 assertions/s