proxygen does not support http methods COPY, MOVE, etc

HHVM with proxygen transport does not support http methods COPY, MOVE, etc and answers with "400 Bad Request", which makes it impossible to implement a WebDAV server using proxygen transport.

Proxygen transport should support arbitrary http methods and map all methods it does not know to POST. (https://github.com/facebook/hhvm/blob/master/hphp/runtime/server/proxygen/proxygen-transport.cpp#L211)
At the moment this happens only for some methods: https://github.com/facebook/hhvm/blob/master/hphp/runtime/server/proxygen/proxygen-transport.cpp#L45

### HHVM Version

3.8.0 up to master

### Standalone code, or other way to reproduce the problem

index.php:
```php