Allow Aria2 downloader to use absolute paths.

[holocronweaver]

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Proposed changes

This should fix #275.

The downloader for aria2 has a bug which prevents it from supporting absolute paths.

From aria2c manpage (v1.35.0):

-o, --out=<FILE>
              The file name of the downloaded file.  It is always relative  to
              the    directory    given    in    --dir   option.    When   the
              --force-sequential option is used, this option is ignored.

Thus both --dir and --out should be specified together for absolute paths, but instead Aria2Downloader in downloaders.py specifies the full path in --out, causing the working directory to be appended to all paths:

def _create_command(self, url, filename):
        return [self.bin, url, '-o', filename,
                '--check-certificate=false', '--log-level=notice',
                '--max-connection-per-server=4', '--min-split-size=1M']

For example, if you run coursera-dl in working directory /home/user1/ and specify --path /home/user1/downloads, aria2 will download to a directory /home/user1//home/user1/downloads. This is what @FinalTheory reported, and is still the behavior as of the latest coursera-dl on PyPi (0.11.5).

This change is a simple fix, though I suspect it would be easier and more robust to avoid these sorts of issues by changing the _create_command interface to pass in filedir and filename separately.

Testing

Added Aria2 downloader unit tests for relative and absolute paths.

I also tested the script myself using absolute and relative paths on real courses, both cases worked.

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Reviewers

@rbrito