Add env var to write raw HTTP traffic to stderr

[kevinburke]

It can be difficult to see the raw traffic going over the wire, but that's what
you need to write tests that integrate with the twilio-php client - the kind
you use for all of your Holodeck tests!

This isn't exactly what goes over the wire, but it's close enough for the
purposes of testing this library. I tried setting CURLOPT_VERBOSE first, but
that prints a bunch of debugging information in addition to the actual data.

[kevinburke]

Sample output:

POST /2010-04-01/Accounts/AC0baf7698c0dcef44271303e0ab9c576b/IncomingPhoneNumbers.json
User-Agent: twilio-php/5.4.2 (PHP 7.0.13)
Accept-Charset: utf-8
Content-Type: application/x-www-form-urlencoded
Accept: application/json

AreaCode=500

HTTP/1.1 201
Access-Control-Allow-Credentials:  true
Access-Control-Allow-Headers:  Accept, Authorization, Content-Type, If-Match, If-Modified-Since, If-None-Match, If-Unmodified-Since
Access-Control-Allow-Methods:  GET, POST, DELETE, OPTIONS
Access-Control-Allow-Origin:  *
Access-Control-Expose-Headers:  ETag
Content-Type:  application/json; charset=utf-8
Date:  Thu, 01 Dec 2016 18:24:28 GMT
Strict-Transport-Security:  max-age=15768000
Twilio-Request-Duration:  0.171
Twilio-Request-Id:  RQb4148ed5d72c4a7d98b956805b8e742a
X-Powered-By:  AT-5000
X-Shenanigans:  none
Content-Length:  956
Connection:  keep-alive

{"sid": "PN59910f870686f2adefbde2340c7335e3", "account_sid": "AC0baf7698c0dcef44271303e0ab9c576b", "friendly_name": "(500) 555-0006", "phone_number": "+15005550006", "voice_url": null, "voice_method": "POST", "voice_fallback_url": null, "voice_fallback_method": "POST", "voice_caller_id_lookup": false, "date_created": "Thu, 01 Dec 2016 18:24:28 +0000", "date_updated": "Thu, 01 Dec 2016 18:24:28 +0000", "sms_url": "", "sms_method": "POST", "sms_fallback_url": "", "sms_fallback_method": "POST", "address_requirements": "none", "beta": false, "capabilities": {"voice": true, "sms": true, "mms": false}, "status_callback": "", "status_callback_method": "POST", "api_version": "2010-04-01", "voice_application_sid": null, "sms_application_sid": "", "trunk_sid": null, "emergency_status": "Inactive", "emergency_address_sid": null, "uri": "/2010-04-01/Accounts/AC0baf7698c0dcef44271303e0ab9c576b/IncomingPhoneNumbers/PN59910f870686f2adefbde2340c7335e3.json"}

I guess the Authorization header is added somewhere else by curl? Maybe that's for the best

[codejudas]

@kevinburke I like this idea but can we check the environment variable in the constructor and then cache it in the client so we arent checking on every request? I dont think we need to be able to dynamically turn this off or on in the lifecycle of the client.

[kevinburke]

Done

[codejudas]

Awesome, we'll get this into the next release. Thanks!