Expose a func to construct header data

[gh73962]

Hi,your ws package helped me a lot! Thank you!

I use this netpoll replace mailru/easygo/netpoll.

I need a func to construct header data like this

Thank again

=== RUN TestWriteHeader === RUN TestWriteHeader/#0 === RUN TestWriteHeader/#1 === RUN TestWriteHeader/#2 === RUN TestWriteHeader/#3 --- PASS: TestWriteHeader (0.00s) --- PASS: TestWriteHeader/#0 (0.00s) --- PASS: TestWriteHeader/#1 (0.00s) --- PASS: TestWriteHeader/#2 (0.00s) --- PASS: TestWriteHeader/#3 (0.00s) PASS ok github.com/gobwas/ws 0.002s