MultiHttpClient: Don't relay the end-of-headers line

MultiHttpClient: Don't relay the end-of-headers line

The callback registered by CURLOPT_HEADERFUNCTION is called for the
empty line that separates the headers from the body, as well as all the
actual headers. In this case, the $header string will be "\r\n".

It turns out that HHVM ignores a call to header() when passed a string
that's empty after trimming whitespace, while Zend PHP only ignores the
call when the string is empty before trimming whitespace. This later
causes problems when headers_list() is used expecting all strings
returned to contain a colon.

Bug: T216086
Change-Id: I07937b17beb06788166266fbb1ea1bbf456761e3