Manage index spark columns instead of index spark column names.

[ueshin]

Manages index spark columns instead of index spark column names as the same as data spark columns.

[codecov-io]

Codecov Report

Merging #1944 (5abd90d) into master (809ed86) will increase coverage by 0.34%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##           master    #1944      +/-   ##
==========================================
+ Coverage   94.16%   94.51%   +0.34%     
==========================================
  Files          41       45       +4     
  Lines       10055    10676     +621     
==========================================
+ Hits         9468    10090     +622     
+ Misses        587      586       -1     

Impacted Files	Coverage Δ
databricks/koalas/base.py	96.89% <ø> (ø)
databricks/koalas/utils.py	95.71% <ø> (ø)
databricks/koalas/window.py	95.89% <ø> (ø)
databricks/koalas/namespace.py	84.23% <84.61%> (ø)
databricks/koalas/frame.py	96.66% <97.29%> (-0.01%)	⬇️
databricks/koalas/accessors.py	93.46% <100.00%> (ø)
databricks/koalas/groupby.py	91.42% <100.00%> (ø)
databricks/koalas/indexes.py	96.95% <100.00%> (+0.10%)	⬆️
databricks/koalas/indexing.py	92.62% <100.00%> (ø)
databricks/koalas/internal.py	96.47% <100.00%> (+0.01%)	⬆️
... and 5 more

---

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 809ed86...5abd90d. Read the comment docs.

[itholic]

Nice work!!

[HyukjinKwon]

+1, nice.

[xinrong-meng]

Awesome! Thank you!

[ueshin]

Thanks! I'd merge this now.