In the http/tests/xmlhttprequest/methods.html test, 

GET request doesn't send Content-Length field. The test log of 'GET(null bytes)' indicates no Content-Length header is sent.

> GET(""): GET(null bytes), Content-Type: null
> GET(null): GET(null bytes), Content-Type: null
> GET("123"): GET(null bytes), Content-Type: null

But http/tests/xmlhttprequest/methods-expected.txt, HEAD seems sending Content-Length header with value of 0.

> HEAD(""): HEAD(0 bytes), Content-Type: null
> HEAD(null): HEAD(0 bytes), Content-Type: null
> HEAD("123"): HEAD(0 bytes), Content-Type: null

As specification defined, HEAD should be as close identical as GET request.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/HEAD
"The HTTP HEAD method requests the headers that are returned if the specified resource would be requested with an HTTP GET method"

Soup and Curl returns (null bytes) which is same with GET requests.