Fix php header

[kevinresol]

Closes #46

[benmerckx]

I don't think this one will pass cookie headers - will write a test for it later. Worst case we'll have to recreate the cookie headers from $_COOKIE. I also think checking getallheaders exists and using it if it does is a better default? Something like this:

if (untyped __call__('function_exists', 'getallheaders')) {
  var raw = php.Lib.hashOfAssociativeArray(untyped __call__('getallheaders'));
  [for (name in raw.keys()) new HeaderField(name, raw[name])];
} else ...

[kevinresol]

I also think checking getallheaders exists and using it if it does is a better default?

Oh sure, I forgot that bit