A content length of zero should also be sent

[shmuelzon]

This is needed since when the content-length header is not sent the clients will
wait for data anyways. Sending a content length of zero will tell the client not
to expect any content and it will close the connection immediately.

Since size_t is unsigned, checking if the content-length is >=0 seems redundant.

[igrr]

For the case when _contentLength == CONTENT_LENGTH_UNKNOWN, this will call

sendHeader("Content-Length", String(contentLength));

where contentLength may be zero. So this will send Content-Length: 0 header even though content length was declared to be unknown.

[shmuelzon]

I've rearranged the conditions a bit so the code (to me) is a bit more readable and also answers the issue Ivan raised.

[Links2004]

the build/test server has failed, try to merge the latest version to trigger a retest.
@igrr may the get.py script need some retry function for download errors.

[shmuelzon]

Done