now injects fake user-agent into requests

[NightMachinery]

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Proposed changes

Coursera has recently added a user-agent check. This commit injects fake user-agent strings to overcome this barrier.

BTW, their behavior is very problematic. Not everyone has the always-on connection and fast devices needed to use their online JS frontend. Here even the power grid is unstable, not to mention the network. Their actions completely disregard people not being as privileged as themselves.

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[coveralls]

Coverage decreased (-0.3%) to 73.305% when pulling d3489b2 on NightMachinary:master into 10ba6b8 on coursera-dl:master.