fix for webpack and node-fetch

[ParcelSolutions]

node-fetch/node-fetch#450

Description

use default in require of node-fetch, so it will work with webpack
webpack is often used to create openwhisk builds.

see for details:
node-fetch/node-fetch#450

Fixes # (issue)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #36 (3dd50c7) into master (4161e43) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #36   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           68        68           
=========================================
  Hits            68        68           

Impacted Files	Coverage Δ
index.js	100.00% <100.00%> (ø)

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[alexkli]

Maybe this is a bit safer (in case we change the node-fetch version in package.json and the .default is no longer there)?

https://github.com/Flagsmith/flagsmith-nodejs-client/pull/21/files#diff-537b33488a6027f3f9f309df24d9af4c6819c635cdae1d650245a259e4f8e335R1