fix: Download as image of dashboard chart did not work

[michael-s-molina]

SUMMARY

Fixes #15872

@junlincc @jinghua-qa

AFTER SCREENSHOTS OR ANIMATED GIF

screen-recording-2021-07-26-at-113701-am_ftfdmunZ.mp4

TESTING INSTRUCTIONS

Check the original issue for instructions.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #15897 (e5777ca) into master (7330aef) will decrease coverage by 0.00%.
The diff coverage is 14.28%.

❗ Current head e5777ca differs from pull request most recent head 965ab99. Consider uploading reports for the commit 965ab99 to get more accurate results

@@            Coverage Diff             @@
##           master   #15897      +/-   ##
==========================================
- Coverage   77.08%   77.08%   -0.01%     
==========================================
  Files         984      984              
  Lines       51787    51793       +6     
  Branches     7036     7040       +4     
==========================================
+ Hits        39920    39923       +3     
- Misses      11642    11645       +3     
  Partials      225      225              

Flag	Coverage Δ
javascript	71.72% <14.28%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset-frontend/src/utils/downloadAsImage.ts	41.37% <0.00%> (-8.63%)	⬇️
superset-frontend/src/components/Select/Select.tsx	73.05% <100.00%> (+0.12%)	⬆️
superset/db_engine_specs/presto.py	90.31% <0.00%> (+0.42%)	⬆️

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[pkdotson]

/testenv up

[github-actions]

@pkdotson Ephemeral environment spinning up at http://54.68.193.161:8080. Credentials are admin/admin. Please allow several minutes for bootstrapping and startup.

[junlincc]

LGTM! thanks for the fix!

[pkdotson]

LGTG!

[github-actions]

Ephemeral environment shutdown and build artifacts deleted.