Add headers. If append is true, headers with the same name will be separated, else overwritten.
Parameters
- headers
associative array containing the additional HTTP headers to add to the messages existing headers
- append
if true, and a header with the same name of one to add exists already, this respective
header will be converted to an array containing both header values, otherwise
it will be overwritten with the new header value
Return Values
Returns TRUE
on success or FALSE
on failure.