Environment details

• OS: linux
• Node.js version: v16.10
• npm version: 7.24.1
• gaxios version: 4.3.2

Steps to reproduce

send a form dat areuqest

const formData = new FormData();
formData.append('test', 123);

gaxios.request({method: POST, data: formData;})

The request is stringiefied because of this line here

formdata is typeof object.

a possible fix will be to check if object is a formdata first and in this case don't touch it?
e.g.

  else if (typeof FormData !== 'undefined' && opts.data instanceof FormData) {
// remove content-type header...  
} else if (typeof opts.data === 'object') { ...

altenative would be to allow skipping the whole transformation step in the validate step.. e.g. via flag?