Various bug fixes and minor improvements

- Fix static responder so it doesn't replace its own headers config,
  and instead replaces the actual response header values
- caddyhttp.ResponseRecorder type optionally buffers response
- Add interface guards to ensure regexp matchers get provisioned
- Use default HTTP port if one is not explicitly set
- Encode middleware writes status code 200 if not written upstream
- Templates and markdown only try to execute on text responses
- Static file server sets Content-Type based on file extension only
  (this whole thing -- MIME sniffing, etc -- needs more configurability)

modules/caddyhttp/fileserver/staticfiles.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"html/template"
 	weakrand "math/rand"
+	"mime"
 	"net/http"
 	"os"
 	"path"
@@ -185,14 +186,21 @@ func (fsrv *FileServer) ServeHTTP(w http.ResponseWriter, r *http.Request) error
 
 	// TODO: Etag
 
-	// do not allow Go to sniff the content-type
 	if w.Header().Get("Content-Type") == "" {
-		w.Header()["Content-Type"] = nil
+		mtyp := mime.TypeByExtension(filepath.Ext(filename))
+		if mtyp == "" {
+			// do not allow Go to sniff the content-type; see
+			// https://www.youtube.com/watch?v=8t8JYpt0egE
+			// TODO: Consider writing a default mime type of application/octet-stream - this is secure but violates spec
+			w.Header()["Content-Type"] = nil
+		} else {
+			w.Header().Set("Content-Type", mtyp)
+		}
 	}
 
 	// let the standard library do what it does best; note, however,
 	// that errors generated by ServeContent are written immediately
-	// to the response, so we cannot handle them (but errors here
+	// to the response, so we cannot handle them (but errors there
 	// are rare)
 	http.ServeContent(w, r, info.Name(), info.ModTime(), file)