fix: DataFrame write shows Future Warning.

[xxjjvxb]

Closes #610

Proposed Changes

use iloc

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• pytest tests completes successfully
• Commit messages are conventional
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (1ec64b7) 90.30% compared to head (12f1973) 90.30%.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #611   +/-   ##
=======================================
  Coverage   90.30%   90.30%           
=======================================
  Files          39       39           
  Lines        3456     3456           
=======================================
  Hits         3121     3121           
  Misses        335      335           

Files Changed	Coverage Δ
...fluxdb_client/client/write/dataframe_serializer.py	98.26% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[bednar]

Thanks for your PR 👍

LGTM 🚀