[PDE-4118] fix(core): Use node-fetch types in BaseHttpResponse

[rnegron]

Fixes #667. As noted in the bug report, using .headers.get() returns a TypeScript error TS2349: This expression is not callable. Type 'String' has no call signatures.:

But testing it out (using vanilla JavaScript) shows that .headers.get() does work (first console.log is response.headers['content-type'], second is response.headers.get('content-type'):

console.log
  undefined

  at perform (lib/creates/movie.js:15:13)

console.log
  application/json; charset=utf-8

  at perform (lib/creates/movie.js:16:13)

[eliangcs]

Looks good!

I didn't know @types/node-fetch exist. I wonder if we could just replace a lot of things, like HttpRequestOptions, BaseHttpResponse, and RawHttpResponse with node-fetch's.

[rnegron]

@eliangcs I looked at replacing some of the other custom types with node-fetch equivalents but there were some subtle differences between them, such as custom methods (like throwForStatus) and properties (like removeMissingValuesFrom).