Add configuration options to allow empty headers

[getaceres]

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.

[kdavisk6]

Do you have a proposal in mind?