PURL server login fails if PURL servers are behind Cloudflare

[jvendetti]

This problem was discovered in our staging environment. The staging PURL server was put behind Cloudflare. After doing so, attempts to create ontologies and submissions at stage.bioontology.org started failing with 500 errors. Tracked the underlying exception down in the unicorn.std.err file on ncbostage-rest2.stanford.edu:

2021-06-23 13:43:02 - NoMethodError - undefined method `split' for nil:NilClass:
	/srv/ncbo/ontologies_api/shared/bundle/ruby/2.6.0/bundler/gems/ontologies_linked_data-71a5ca8f7574/lib/ontologies_linked_data/purl/purl_client.rb:17:in `purl_server_login'
	/srv/ncbo/ontologies_api/shared/bundle/ruby/2.6.0/bundler/gems/ontologies_linked_data-71a5ca8f7574/lib/ontologies_linked_data/purl/purl_client.rb:27:in `create_purl'
	/srv/ncbo/ontologies_api/shared/bundle/ruby/2.6.0/bundler/gems/ontologies_linked_data-71a5ca8f7574/lib/ontologies_linked_data/models/ontology.rb:409:in `save'
	/srv/ncbo/ontologies_api/releases/20210619061350/controllers/ontologies_controller.rb:148:in `create_ontology'
	/srv/ncbo/ontologies_api/releases/20210619061350/controllers/ontologies_controller.rb:49:in `block (2 levels) in <class:OntologiesController>'

[jvendetti]

This change to the infrastructure was reverted, i.e., the staging PURL server is no longer behind Cloudflare. New ontology and submission creation is working again in staging. The desire is to address this issue at some point in the future.

[alexskr]

relates to #169