Invoke headers function for remote uploads

[dschmidt]

Streamline the behavior of local and remote uploads - currently when headers is a function it's serialized to undefined (or rather it's skipped) for remote uploads but for local uploads it's called and the result is serialized.

c.f.

uppy/packages/@uppy/tus/src/index.js

Lines 197 to 199 in 5146907

	if (typeof opts.headers === 'function') {
	opts.headers = opts.headers(file)
	}

uppy/packages/@uppy/xhr-upload/src/index.js

Lines 149 to 153 in 5146907

	if (typeof headers === 'function') {
	opts.headers = headers(file)
	} else {
	Object.assign(opts.headers, this.opts.headers)
	}

There is also

uppy/packages/@uppy/aws-s3/src/index.js

Lines 224 to 226 in 5146907

	if (headers) {
	xhrOpts.headers = headers
	}

Which looks like it should do the same, but the code is a little different and I'm not 100% sure what I'm doing there

[mifi]

Nice catch! preferably we should only do this once (probably initially when setting and validating this.opts?) but the code is a bit messy so I guess we can do a refactor later. For now, should we create a helper function so we don't have to duplicate the logic?

[dschmidt]

tbh I think it's so messy, that it needs a proper refactor and it's not worth adding a helper function

The logic is pretty simple and this way it's easier to directly see what's going on without jumping to the function

tl;dr: I would keep it as it is