Fix bug occurring when trailing newline characters are used with url_mode

[developius]

• trim trailing newline characters from the URL in url_mode (was creating invalid URLs)
• add a default user agent to download files without triggering WAFs etc

Thanks to @vielmetti for helping to identify the issues 👍

Signed-off-by: Finnian Anderson [email protected]

[vielmetti]

In particular, if you invoke colorise with

echo http://example.com/picture.jpg | faas-cli invoke colorise > picture-colored.png

the function will get an argument "http://example.com/picture.jpg\n" from the echo command. The workaround until this patch lands is to use "echo -n".