Are HTTPS headers encrypted?

The whole lot is encrypted^† - all the headers. That's why SSL on vhosts doesn't work too well - you need a dedicated IP address because the Host header is encrypted.

^†The Server Name Identification (SNI) standard means that the hostname may not be encrypted if you're using TLS. Also, whether you're using SNI or not, the TCP and IP headers are never encrypted. (If they were, your packets would not be routable.)

The headers are entirely encrypted. The only information going over the network 'in the clear' is related to the SSL setup and D/H key exchange. This exchange is carefully designed not to yield any useful information to eavesdroppers, and once it has taken place, all data is encrypted.