cleanup: merge httpheader and httpfailure into model (#758)

These two small packages could easily be merged into the model
package, since they're clearly model-like packages.

Part of https://github.com/ooni/probe/issues/2115

internal/model/http.go

@@ -0,0 +1,33 @@
+package model
+
+//
+// Common HTTP definitions.
+//
+
+// Headers we use for measuring.
+const (
+	// HTTPHeaderAccept is the Accept header used for measuring.
+	HTTPHeaderAccept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"
+
+	// HTTPHeaderAcceptLanguage is the Accept-Language header used for measuring.
+	HTTPHeaderAcceptLanguage = "en-US,en;q=0.9"
+
+	// HTTPHeaderUserAgent is the User-Agent header used for measuring. The current header
+	// is 13.7% of the browser population as of May 20, 2022 according to the
+	// https://techblog.willshouse.com/2012/01/03/most-common-user-agents/ webpage.
+	HTTPHeaderUserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4896.127 Safari/537.36"
+)
+
+// Additional strings used to report HTTP errors. They're currently only used by
+// experiment/whatsapp but may be used by more experiments in the future. They must
+// be addressable (i.e., var and not const) because experiments typically want to
+// take their addresses to fill fields with `string|null` type.
+var (
+	// HTTPUnexpectedStatusCode indicates that we re not getting
+	// the expected (range of) HTTP status code(s).
+	HTTPUnexpectedStatusCode = "http_unexpected_status_code"
+
+	// HTTPUnexpectedRedirectURL indicates that the redirect URL
+	// returned by the server is not the expected one.
+	HTTPUnexpectedRedirectURL = "http_unexpected_redirect_url"
+)