Check cached sources with upstream after the cache TTL (closes #69) (#350)

internal/httpfetcher/mock.go

@@ -24,9 +24,10 @@ func NewMock(fsys fs.FS) *MockFetcher {
 	return &MockFetcher{fs: fsys}
 }
 
-// Fetch retrieves the source's content from the mock filesystem.
+// Fetch retrieves the source's content from the mock filesystem. It ignores
+// ifNoneMatch and ifModifiedSince, and always sends the content.
 func (m *MockFetcher) Fetch(
-	ctx context.Context, source imagerequest.Source,
+	ctx context.Context, source imagerequest.Source, _, _ string,
 ) (*FetchResult, error) {
 	// Check context cancellation
 	select {