Add configuration options to allow empty headers

Hello, I'm trying to implement a client for a Web Service using Feign. The problem with web services is that they always require sending a SOAPAction even if that header is empty. Right now, the Feign template removes empty headers as per documentation:

`
Request Headers Expansion
Headers and HeaderMap templates follow the same rules as Request Parameter Expansion with the following alterations:

Unresolved expressions are omitted. If the result is an empty header value, the entire header is removed.
`

This makes impossible to call this kind of web services using Feign.

It would be nice to have a configuration option in the Feign builder to enable sending empty headers if present.

Start by tracing Feign's request-header expansion entry point and the builder configuration described in the issue, focusing on where empty header values are removed. Done means a builder option can preserve explicitly empty headers for SOAPAction while retaining current removal behavior by default, with tests covering both modes.