Support of SoapHeader

Hi,

It would be great, if there could be some support of SoapHeaders when you are using Feign for legacy Soap calls. I just recently came to legacy API, where it was mandatory.

And having something like this would be absolutely amazing as headers might be marshalled objects.

@RequestLine("POST /getObject")
@Headers({
  "SOAPAction: getObject",
  "Content-Type: text/xml"
})
MyJaxbObjectResponse getObject(@SoapHeader MyJaxbHeader header, MyJaxbObjectRequest request);

Second best thing would be method annotation where you could define per method interceptor @SoapHeader(MyJaxbHeaderInterceptor.class)

The simplest solution (which I chose as it is the simplest one and it fit my need) would be to add interceptor in builder (to encoder).

I created copy of SoapEncoder SoapEncoderWithHeader(JAXBContextFactory jaxbContextFactory, MyJaxbHeaderInterceptor headerInterceptor) and I only had to add following two lines in encode method:

Marshaller headerMarshaller = jaxbContextFactory.createMarshaller(interceptor.getHeaderClass());
headerMarshaller.marshal(interceptor.createHeader(), soapMessage.getSOAPHeader());

Start by reviewing SoapEncoder and the Feign builder's encoder configuration, then compare the requested parameter annotation, method annotation, and encoder-interceptor approaches. Done means SOAP headers can be marshalled into the SOAP message while preserving the existing request example's behavior.