Allow specifying multiples values of the same header name on separate lines

```
Currently we require that multiple values of the same header name be specified 
on a single line (which is always legal with a comma separator). This is 
inconvenient, allow them to be spread across lines.

e.g. 

Cookie: name=x, place=y

can become

Cookie: name=x
Cookie: place=y
```

Original issue reported on code.google.com by `stephen....@gmail.com` on 24 Jun 2014 at 8:38
