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;

vendor/github.com/gorilla/handlers/compress.go

@@ -0,0 +1,148 @@
+// Copyright 2013 The Gorilla Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package handlers
+
+import (
+	"compress/flate"
+	"compress/gzip"
+	"io"
+	"net/http"
+	"strings"
+)
+
+type compressResponseWriter struct {
+	io.Writer
+	http.ResponseWriter
+	http.Hijacker
+	http.Flusher
+	http.CloseNotifier
+}
+
+func (w *compressResponseWriter) WriteHeader(c int) {
+	w.ResponseWriter.Header().Del("Content-Length")
+	w.ResponseWriter.WriteHeader(c)
+}
+
+func (w *compressResponseWriter) Header() http.Header {
+	return w.ResponseWriter.Header()
+}
+
+func (w *compressResponseWriter) Write(b []byte) (int, error) {
+	h := w.ResponseWriter.Header()
+	if h.Get("Content-Type") == "" {
+		h.Set("Content-Type", http.DetectContentType(b))
+	}
+	h.Del("Content-Length")
+
+	return w.Writer.Write(b)
+}
+
+type flusher interface {
+	Flush() error
+}
+
+func (w *compressResponseWriter) Flush() {
+	// Flush compressed data if compressor supports it.
+	if f, ok := w.Writer.(flusher); ok {
+		f.Flush()
+	}
+	// Flush HTTP response.
+	if w.Flusher != nil {
+		w.Flusher.Flush()
+	}
+}
+
+// CompressHandler gzip compresses HTTP responses for clients that support it
+// via the 'Accept-Encoding' header.
+//
+// Compressing TLS traffic may leak the page contents to an attacker if the
+// page contains user input: http://security.stackexchange.com/a/102015/12208
+func CompressHandler(h http.Handler) http.Handler {
+	return CompressHandlerLevel(h, gzip.DefaultCompression)
+}
+
+// CompressHandlerLevel gzip compresses HTTP responses with specified compression level
+// for clients that support it via the 'Accept-Encoding' header.
+//
+// The compression level should be gzip.DefaultCompression, gzip.NoCompression,
+// or any integer value between gzip.BestSpeed and gzip.BestCompression inclusive.
+// gzip.DefaultCompression is used in case of invalid compression level.
+func CompressHandlerLevel(h http.Handler, level int) http.Handler {
+	if level < gzip.DefaultCompression || level > gzip.BestCompression {
+		level = gzip.DefaultCompression
+	}
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	L:
+		for _, enc := range strings.Split(r.Header.Get("Accept-Encoding"), ",") {
+			switch strings.TrimSpace(enc) {
+			case "gzip":
+				w.Header().Set("Content-Encoding", "gzip")
+				w.Header().Add("Vary", "Accept-Encoding")
+
+				gw, _ := gzip.NewWriterLevel(w, level)
+				defer gw.Close()
+
+				h, hok := w.(http.Hijacker)
+				if !hok { /* w is not Hijacker... oh well... */
+					h = nil
+				}
+
+				f, fok := w.(http.Flusher)
+				if !fok {
+					f = nil
+				}
+
+				cn, cnok := w.(http.CloseNotifier)
+				if !cnok {
+					cn = nil
+				}
+
+				w = &compressResponseWriter{
+					Writer:         gw,
+					ResponseWriter: w,
+					Hijacker:       h,
+					Flusher:        f,
+					CloseNotifier:  cn,
+				}
+
+				break L
+			case "deflate":
+				w.Header().Set("Content-Encoding", "deflate")
+				w.Header().Add("Vary", "Accept-Encoding")
+
+				fw, _ := flate.NewWriter(w, level)
+				defer fw.Close()
+
+				h, hok := w.(http.Hijacker)
+				if !hok { /* w is not Hijacker... oh well... */
+					h = nil
+				}
+
+				f, fok := w.(http.Flusher)
+				if !fok {
+					f = nil
+				}
+
+				cn, cnok := w.(http.CloseNotifier)
+				if !cnok {
+					cn = nil
+				}
+
+				w = &compressResponseWriter{
+					Writer:         fw,
+					ResponseWriter: w,
+					Hijacker:       h,
+					Flusher:        f,
+					CloseNotifier:  cn,
+				}
+
+				break L
+			}
+		}
+
+		h.ServeHTTP(w, r)
+	})
+}