Improve reverse proxy support

Should address edge cases and close #224 -- subject to field testing.

Does require the NGINX deployments use the following:

proxy_pass <http://documize-url:5001>;
proxy_set_header Host $http_host;
proxy_set_header X-Forwarded-For $remote_addr;

core/request/url.go

@@ -0,0 +1,29 @@
+// Copyright 2016 Documize Inc. <legal@documize.com>. All rights reserved.
+//
+// This software (Documize Community Edition) is licensed under
+// GNU AGPL v3 http://www.gnu.org/licenses/agpl-3.0.en.html
+//
+// You can operate outside the AGPL restrictions by purchasing
+// Documize Enterprise Edition and obtaining a commercial license
+// by contacting <sales@documize.com>.
+//
+// https://documize.com
+
+// Package request provides HTTP request parsing functions.
+package request
+
+import (
+	"net/http"
+	"strings"
+)
+
+// IsSSL returns true if Referer header contains "https".
+// If Referer header is empty we look at r.TLS setting.
+func IsSSL(r *http.Request) bool {
+	rf := r.Referer()
+	if len(rf) > 1 {
+		return strings.HasPrefix(rf, "https")
+	}
+
+	return r.TLS != nil
+}