add type=application/geo+json to prev and next links

[philvarner]

Related Issue(s): #256

Proposed Changes:

1. add type=application/geo+json to prev and next link relations

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.

[m-mohr]

Just realized a day ago, but is it in a conflict that the type is geo+json instead of just JSON and errors being given as plain json?

[philvarner]

It's not a conflict. This is more just an advertisement that (1) if you sent an Accept header with this value (or no Accept header) it will respond with a 200 and that type as the body. Any non-200 response isn't expected to return the same type.