New: Add 'accept' header option on validator

[ahankinson]

For IIIF services that provide both v2 and v3 manifests at the same URL, the validator needs to send an appropriate 'accept' header to ensure it is returning the appropriate IIIF version response.

This PR adds a checkbox to the validator to optionally include an 'Accept' header in the request. The header is set according to the selected IIIF version, which makes it possible to request v3 manifests from a 'default' v2 service, or vice-versa.

[coveralls]

Coverage decreased (-0.8%) to 81.752% when pulling 3412b7e6266f2aeb6b51589d022f5c294b4abbd5 on bodleian:add-accept-header into bd49884cfe42fbd83b8b2fe0dba88c71372c69b8 on IIIF:master.

[ahankinson]

I think the CI failure is during installation for Python 2.7, and not because of this change.

[glenrobson]

Thanks Andrew, I’ll have a proper look but I think the python 2.7 issue is due to the version of Pillow. I’ve got a fix in a branch I’m working on to improve the error messages and I’ll merge that first before this so the fix is in the master branch.

[ahankinson]

I just noticed that this would fix #74 as well