'Gets a header value.
Args:
header_name: HTTP header name to look for.
Returns:
A header value that corresponds to header_name. If more than one such
value is in self, one of the values is selected arbitrarily, and
returned. The selection is not deterministic.'

def Get(self, header_name):

    for name in self:
        if (name.lower() == header_name.lower()):
            return self[name]
