locale negotiation is incorrect for i18n routing

# Bug report ## Describe the bug If I send Accept-Language: "fr-XX,en" and I have available locales fr, en, it should give me fr instead of en (current behavior) ## To Reproduce Steps to reproduce the behavior, please provide code snippets or a repository: 1. Go to '...' 2. Click on '....' 3. Scroll down to '....' 4. See error ## Expected behavior Should be routed to fr ## Screenshots If applicable, add screenshots to help explain your problem. ## System information - OS: [e.g. macOS, Windows] - Browser (if applies) [e.g. chrome, safari] - Version of Next.js: [e.g. 6.0.2] - Version of Node.js: [e.g. 10.10.0] ## Additional context Add any other context about the problem here.