chore(view_api): return application/json as content-type for api/v1/form_data endpoint

[zephyring]

…views/api

SUMMARY

1. return content-type application/json for api/v1/form_data endpoint.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• 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 #24758 (2f089be) into master (7675e0d) will increase coverage by 0.00%.
The diff coverage is 58.69%.

❗ Current head 2f089be differs from pull request most recent head ee855db. Consider uploading reports for the commit ee855db to get more accurate results

@@           Coverage Diff           @@
##           master   #24758   +/-   ##
=======================================
  Coverage   68.94%   68.95%           
=======================================
  Files        1901     1902    +1     
  Lines       73898    73939   +41     
  Branches     8175     8176    +1     
=======================================
+ Hits        50951    50981   +30     
- Misses      20834    20845   +11     
  Partials     2113     2113           

Flag	Coverage Δ
hive	54.14% <17.85%> (-0.03%)	⬇️
mysql	79.19% <46.42%> (-0.01%)	⬇️
postgres	79.29% <46.42%> (-0.01%)	⬇️
presto	54.03% <17.85%> (-0.03%)	⬇️
python	83.35% <85.71%> (+0.02%)	⬆️
sqlite	77.86% <46.42%> (-0.01%)	⬇️
unit	54.96% <75.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
...src/components/ColumnTypeLabel/ColumnTypeLabel.tsx	100.00% <ø> (ø)
...d/packages/superset-ui-chart-controls/src/index.ts	100.00% <ø> (ø)
...-chart-controls/src/sections/advancedAnalytics.tsx	14.28% <ø> (ø)
...rset-ui-chart-controls/src/sections/chartTitle.tsx	100.00% <ø> (ø)
...legacy-plugin-chart-partition/src/controlPanel.tsx	25.00% <ø> (ø)
...gins/legacy-plugin-chart-rose/src/controlPanel.tsx	50.00% <ø> (ø)
...gins/legacy-preset-chart-nvd3/src/NVD3Controls.tsx	100.00% <ø> (ø)
.../BigNumber/BigNumberWithTrendline/controlPanel.tsx	16.66% <ø> (ø)
...s/plugin-chart-echarts/src/Funnel/controlPanel.tsx	66.66% <ø> (ø)
...ns/plugin-chart-echarts/src/Gauge/controlPanel.tsx	66.66% <ø> (ø)
... and 24 more

... and 1 file with indirect coverage changes

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[eschutho]

nice!