Failed Assert - When we finish downloading in paid speedup mode

[mnaamani]

Assertion failed: (_state == BuyingState::downloading), function removeSeller, file /Users/mokhtar/.electron-gyp/.conan/data/ProtocolSession/0.0.0/joystream/testing/package/959eb5500db3f2e261384c68e583baa249b46c9e/include/protocol_session/detail/Buying.cpp, line 646.

[mnaamani]

This could be for similar reason to issue #13

[mnaamani]

I think the fix in #15 has also resolved this problem.