Don't use cached size for file responses

Fixes #7.

src/DocumentRoot.php

@@ -380,13 +380,18 @@ private function checkPreconditions(Request $request, int $mtime, string $etag):
     private function doNonRangeResponse(Internal\FileInformation $fileInfo): \Generator
     {
         $headers = $this->makeCommonHeaders($fileInfo);
-        $headers["Content-Length"] = (string) $fileInfo->size;
         $headers["Content-Type"] = $this->selectMimeTypeFromPath($fileInfo->path);
 
-        if (isset($fileInfo->buffer)) {
+        if ($fileInfo->buffer !== null) {
+            $headers["Content-Length"] = (string) $fileInfo->size;
+
             return new Response(Status::OK, $headers, new InMemoryStream($fileInfo->buffer));
         }
 
+        // Don't use cached size if we don't have buffered file contents,
+        // otherwise we get truncated files during development.
+        $headers["Content-Length"] = (string) $this->filesystem->size($fileInfo->path);
+
         $handle = yield $this->filesystem->open($fileInfo->path, "r");
 
         $response = new Response(Status::OK, $headers, $handle);