Fix merge to support multi-index columns.

[ueshin]

No description provided.

[codecov-io]

Codecov Report

Merging #825 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #825      +/-   ##
==========================================
+ Coverage   94.36%   94.37%   +<.01%     
==========================================
  Files          32       32              
  Lines        5855     5862       +7     
==========================================
+ Hits         5525     5532       +7     
  Misses        330      330

Impacted Files	Coverage Δ
databricks/koalas/internal.py	95.87% <100%> (+0.01%)	⬆️
databricks/koalas/frame.py	96.91% <100%> (+0.01%)	⬆️

---

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 2b80dcf...2fb3308. Read the comment docs.

[softagram-bot]

Softagram Impact Report for pull/825 (head commit: 2fb3308)

⭐ Change Overview

(Open in Softagram Desktop for full details)

📄 Full report

• Permalink: Full report for pull/825

Impact Report explained. Give feedback on this report to [email protected]

[HyukjinKwon]

Haven't taken a close look yet but seems fine in general

[ueshin]

Thanks! merging.