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

All modified and coverable lines are covered by tests ✅

Project coverage is 99.47%. Comparing base (b89e19a) to head (ad03d65).
Report is 18 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1115      +/-   ##
==========================================
- Coverage   99.61%   99.47%   -0.14%     
==========================================
  Files           5        7       +2     
  Lines         520      575      +55     
  Branches      147      165      +18     
==========================================
+ Hits          518      572      +54     
- Misses          2        3       +1     

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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 🚀

[kevinpeno]

@jonathanong should be gtg. Codecov is bitching, but the related missed line is not related to the changes made in this PR. It is likely the methods added to this PR pushed the file over its limits. I think we should ignore it for this PR.

[iwanofski]

I couldn't see this update because this was done om my "stolen" account :) Great to see this merged.