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/encode/encode.go

@@ -157,7 +157,11 @@ func (rw *responseWriter) init() {
 		rw.Header().Set("Content-Encoding", rw.encodingName)
 	}
 	rw.Header().Del("Accept-Ranges") // we don't know ranges for dynamically-encoded content
-	rw.ResponseWriter.WriteHeader(rw.statusCode)
+	status := rw.statusCode
+	if status == 0 {
+		status = http.StatusOK
+	}
+	rw.ResponseWriter.WriteHeader(status)
 }
 
 // Close writes any remaining buffered response and