Fix the issue with `HttpHeaders.setHost()` method not deleting the host

[bivo]

This PR fixes a bug in org.springframework.http.HttpHeaders class. It was not possible to delete/clear the Host header before this fix.

This bug was first introduced in this commit (in 2018): https://github.com/spring-projects/spring-framework/commit/5bbbc82e19e4c2ed443ce955949137ec8c0a2e02#diff-e2d6218e6585f8b7e32682f6f8f7aed22dbd76d862ec92e86e0902243889556aR1020

After that commit, the deletion logic line in setHost() method started to look like this: remove(HOST, null) Although for all other methods/headers it looks like this: remove(CONTENT_TYPE), remove(ETAG), etc.

And for setHost() method it also should be the same: remove(HOST)

I've also added a test for this case. Feel free to rollback my 1-line fix in HttpHeaders and check that Host header is not cleared indeed.