Add Issue channel access token API

[drillbits]

https://developers.line.me/en/docs/messaging-api/reference/#issue-channel-access-token

[sugyan]

Sorry for the late response.
We were carefully considering whether to include these methods in this SDK.
I tried issuing a pull-request to follow the latest changes, so please check it. 🙇

drillbits#1

[linxGnu]

Please follow practice:

if err != nil {
    return nil, err
}
defer closeResponse(res)

This practice prevent malform payload response from server. And make client connection reusable.

[drillbits]

Thanks for your review. I fixed it!

[drillbits]

@sugyan
Sorry for the late response, too...
I merged drillbits#1 and synced with the upstream.

[codecov-io]

Codecov Report

Merging #63 into master will increase coverage by 0.2%.
The diff coverage is 79.59%.

@@            Coverage Diff            @@
##           master      #63     +/-   ##
=========================================
+ Coverage   72.11%   72.31%   +0.2%     
=========================================
  Files          23       24      +1     
  Lines        1757     1806     +49     
=========================================
+ Hits         1267     1306     +39     
- Misses        413      418      +5     
- Partials       77       82      +5

Impacted Files	Coverage Δ
linebot/response.go	66.93% <50%> (-1.17%)	⬇️
linebot/client.go	73.75% <66.66%> (-0.58%)	⬇️
linebot/oauth.go	88.57% <88.57%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e039956...13e4bb1. Read the comment docs.

[sugyan]

LGTM 🙆‍♂

[tkgauri]

#154

[k2wanko]

LGTM