response.redirect: extract special-cased back, fixes #904

[fl0w]

The simple solution to drop special-cased 'back' in response.redirect.
I can't label but should be labeled as version-major.

This PR does not make use of Symbol as purposed in #904.

Edit
If this solution is acceptable, a deprecation should be added to Koa 2 on 'back' use.

[codecov]

Codecov Report

Merging #1115 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1115   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files           5       5           
  Lines         393     398    +5     
======================================
+ Hits          393     398    +5

Impacted Files	Coverage Δ
lib/context.js	100% <ø> (ø)	⬆️
lib/response.js	100% <100%> (ø)	⬆️
lib/request.js	100% <0%> (ø)	⬆️
lib/application.js	100% <0%> (ø)	⬆️

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[fl0w]

I'm assuming this is a travis-cache thingie that fails, can't reproduce it locally.

$ git rev-parse --abbrev-ref HEAD
refactor-back
$ node -v
v9.3.0
$ npm -v
5.6.0
$ npm view koa version
2.4.1
$ npm t

# abbreviated jest output

Test Suites: 66 passed, 66 total
Tests:       298 passed, 298 total
Snapshots:   0 total
Time:        2.265s
Ran all test suites.

[jonathanong]

👍

let's add this as a feature + deprecation message, then a separate PR to remove it in the next major version

[fl0w]

@jonathanong updated, and should be safe as a semver-minor merge. Just take a quick look at the deprecation message before doing so.

[3imed-jaberi]

LGTM, I believe that should be merged this PR 🚀