fix: multipart/form-data header issues with node/js fetch targets

[erunion]

Since #227 has us a bit spooked we've started testing other targets that may also have issues with multipart/form-data requests and found that both of the fetch targets for Node and JS are not currently transferring data properly because the custom header that is being added is clashing with the auto-generated header from the FormData API and form-data module.

You can see this problem in action with the multipart-form-data snippet against https://httpbin.org/anything:

const FormData = require('form-data');
const fetch = require('node-fetch');
const formData = new FormData();

formData.append('foo', 'bar');

let url = 'https://httpbin.org/anything';

const options = {
  method: 'POST',
  headers: {'Content-Type': 'multipart/form-data; boundary=---011000010111000001101001'}
};

options.body = formData;

fetch(url, options)
  .then(res => res.json())
  .then(json => console.log(json))
  .catch(err => console.error('error:' + err));

{
  args: {},
  data: '',
  files: {},
  form: {},
  headers: {
    Accept: '*/*',
    'Accept-Encoding': 'gzip,deflate',
    'Content-Length': '161',
    'Content-Type': 'multipart/form-data; boundary=---011000010111000001101001',
    Host: 'httpbin.org',
    'User-Agent': 'node-fetch/1.0 (+https://github.com/bitinn/node-fetch)',
    'X-Amzn-Trace-Id': 'Root=1-61428408-5a6abe107e16c34466e8e6ba'
  },
  json: null,
  method: 'POST',
  origin: 'REDACTED',
  url: 'https://httpbin.org/anything'
}

After removing the multipart/form-data header:

{
  args: {},
  data: '',
  files: {},
  form: { foo: 'bar' },
  headers: {
    Accept: '*/*',
    'Accept-Encoding': 'gzip,deflate',
    'Content-Length': '161',
    'Content-Type': 'multipart/form-data;boundary=--------------------------635295776437093815404580',
    Host: 'httpbin.org',
    'User-Agent': 'node-fetch/1.0 (+https://github.com/bitinn/node-fetch)',
    'X-Amzn-Trace-Id': 'Root=1-6142841e-4c789faa2a1d423452674f13'
  },
  json: null,
  method: 'POST',
  origin: '98.42.248.146',
  url: 'https://httpbin.org/anything'
}

[erunion]

I renamed this variable to reqOpts so it's not overwriting the options argument of the target.

[reynolek]

Thanks @erunion -- I figured there might be a few other places multipart was broken.