Send configured headers from the GeoBlacklight harvester

[thatbudakguy]

Closes #210

Headers can be configured globally or per site, and are sent with every request a harvest makes:

GeoCombine::GeoBlacklightHarvester.configure do
  {
    headers: { 'User-Agent' => 'GeoCombine harvester (you@example.edu)' }, # all sites
    earthworks: {
      host: 'https://earthworks.stanford.edu/',
      headers: { 'foo' => 'bar' }, # earthworks only
      params: { f: { schema_provider_s: ['Stanford'] } }
    }
  }
end

If you configure headers at the global level, they get merged with the per-site header configuration.