Add user-agent headers to HTTP requests to satisfy new Canvas requirement

[chrispittman]

In mid-January, the Canvas LMS will stop accepting API requests or LTI 1.1 grade-passback requests that don't include a User-Agent header. (details at https://community.canvaslms.com/t5/Canvas-LMS-Blog/Enforcing-User-Agent-Header-for-Canvas-API-Requests/ba-p/658205)

Neither of the current ClientInterface implementations include this header... adding a manual user-agent seems pretty straightforward, though. Here's two fixes that test well for me:

CurlClient->send() : add curl_setopt($ch, CURLOPT_USERAGENT, 'ceLTIc/LTI PHP Library');

StreamClient->send(): add $message->requestHeaders[] = 'User-Agent: ceLTIc/LTI PHP Library';

[jonespm]

Thanks for filing! I saw this issue to make it configurable in php.ini but looks like it was never implemented php/php-src#6834

It might be useful to have this agent value configurable incase Canvas ever enforced specific UA's but for now if you send any value it will be accepted so that would be sufficient.

[chrispittman]

Probably worth noting that I've only tested that these two additions successfully add the header when using the ClientInterface->send() function directly. I haven't yet had a chance to test a full OAuth LTI 1.1 grade-passback request. I don't expect that'd pose any additional problems; the new user-agent header should be excluded when computing OAuth's HMAC signature.

[spvickers]

@chrispittman, thanks for reporting this issue.