ArXiv fetcher should support http urls

[koppor]

1. Goto https://arxiv.org/abs/1802.01168
2. Klick on "copy link location" at "Cite as: arXiv:1802.01168"
3. New Entry
4. From ArXiv
5. Paste https://arxiv.org/abs/1802.01168
6. Click "Generate"
7. Error dialog appears.

Similar functionality as the DOI parser should be implemented.

[Siedlerchr]

Fixed in JabRef#4367