Fix parsing headers: short and long version of "Via"

[denisolyanyuk]

When I was trying to work with 8x8 provider, I was getting 400 Malformed Request

I Found out that it's because short version of header "Via" - even if we get "v" to headers we adding "Via".

Here is small fix

[JonnoFTW]

Suggested change

	if key == "Via" or key == "v":
	headers[key].append(value)
	if key not in headers.keys():
	headers[key] = value
	if key in headers:
	headers[key].append(value)
	else:
	headers[key] = value

[TAbdiukov]

@denis-olyanyuk @JonnoFTW This is a useful patch, however, I implemented a more thorough implementation of short tags support (in a more planned out manner) in rfcvoip