Use HTTP POST for Solr requests

Requires: Blacklight 4.6.3 or higher

To use HTTP POST in a specific controller (e.g., CatalogController):

configure_blacklight do |config|
  config.http_method = :post
end

To set HTTP POST as the default (i.e, for all instances of Blacklight::Configuration) add the following to a Rails initializer (or similar mechanism):

Blacklight::Configuration.default_values[:http_method] = :post

NOTE: This solution does not change the behavior of ActiveFedora::SolrService.