This commit adds missing status codes:

* 102 - Processing
* 207 - Multi-Status
* 208 - Already Reported
* 226 - IM Used
* 308 - Permanent Redirect
* 422 - Unprocessable Entity
* 423 - Locked
* 424 - Failed Dependency
* 426 - Upgrade Required
* 506 - Variant Also Negotiates
* 507 - Insufficient Storage
* 508 - Loop Detected
* 510 - Not Extended
* 511 - Network Authentication Required

Change-Id: Ife0e5b064f4b1e3542d2fd41abc9e7b1e410b644
Reviewed-on: https://go-review.googlesource.com/23090Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>