Refactoring: variable names, adding tests and http constants

gateway/handlers/alerthandler.go

gateway/handlers/cors.go

gateway/handlers/create_handler.go

gateway/handlers/forwarding_proxy.go

gateway/handlers/proxy.go

gateway/handlers/queueproxy.go

gateway/handlers/reader.go

gateway/metrics/prometheus_query.go

gateway/metrics/swarmwatcher.go

gateway/plugin/external.go

gateway/server.go

gateway/tests/alerthandler_test.go

gateway/tests/cors_test.go

gateway/tests/proxy_test.go

gateway/tests/reader_test.go

gateway/tests/registryauth_test.go

gateway/tests/resources_test.go

gateway/tests/unmarshall_test.go

gateway/types/http_adapter.go