[JS] Fix cookie support in Node.js client

[delenius]

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Description of the PR

After a change to superagent, the cookie support broke, because they renamed the attachCookies and saveCookies methods, adding underscores in the beginning.

Technically, these methods are considered "private", and this may break again. But for now, this is the easiest fix.

Technical committee:
@CodeNinjai @frol @cliffano

[wing328]

@delenius thanks for the PR. Do we need to update the dependency accordingly?

openapi-generator/modules/openapi-generator/src/main/resources/Javascript/es6/package.mustache

Line 16 in 21777f2

"superagent": "3.7.0"

openapi-generator/modules/openapi-generator/src/main/resources/Javascript/package.mustache

Line 14 in 21777f2

"superagent": "3.7.0"

[wing328]

[Optional] Maybe we should take this opportunity to consider upgrading superagent to the latest version.

[wing328]

@delenius thanks again for the PR, which is included in the v3.3.0 minor release: https://twitter.com/oas_generator/status/1046941449609068544