make name parameter in HTTP\Header mandatory

[samsonasik]

if the name is not mandatory, when we create a HTTP\Header and call it as string from __toString(), we will get the ": " value, which seems incorrect as message header.

[lonnieezell]

Hmm. I was thinking there were some headers that only had a single value but I'm not able to find this now, so I"ll merge this for now with the caveat that if I find something along those lines in the future we might have to re-evaluate.