[core] support empty CSRF token

[williaster]

🏆 Enhancements

• This PR adds support for csrf_token='' to SupersetClient, to support the case when the Superset Flask app sets WTF_CSRF_ENABLED = False. This is valid configuration and is used in e.g., cypress tests, and when this is set to False, the superset/csrf_token endpoint returns a 200 with an empty string. Without this support, Cypress tests fail when integrating SupersetClient.

🏠 Internal

• removes unneeded polyfill (used previously as another method for encoding POST payload form data)
• adds a test to ensure that parseResponse rejects when response.ok=false

@kristw @michellethomas @graceguo-supercat @conglei

[codecov]

Codecov Report

Merging #7 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #7      +/-   ##
==========================================
+ Coverage    98.5%   98.52%   +0.02%     
==========================================
  Files           7        7              
  Lines          67       68       +1     
==========================================
+ Hits           66       67       +1     
  Misses          1        1

Impacted Files	Coverage Δ
packages/superset-ui-core/src/callApi/callApi.js	100% <ø> (ø)	⬆️
...ages/superset-ui-core/src/callApi/parseResponse.js	100% <100%> (+11.11%)	⬆️
packages/superset-ui-core/src/SupersetClient.js	97.56% <100%> (-2.44%)	⬇️

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[kristw]

lgtm