Prior to this commit, the `header(String, String)` method in the
ServerHttpRequest.Builder API actually added a header value instead of
setting or overriding a header value. Since this conflicted with the
stated behavior in the Javadoc as well as the original intention of the
method, we have decided to introduce an overloaded variant
`header(String, String...)` which accepts a var-args list of header
values to set or override.
In addition, this commit deprecates the existing `header(String, String)`
method for removal in Spring Framework 5.2.
In order not to be a breaking change for custom implementations of the
builder API, this commit implements the new `header(String, String...)`
method as an interface `default` method, with the intent to remove the
default implementation in Spring Framework 5.2
closes gh-23333
9655fc5007