Support retention of existing HTTP headers when adding the Authorization header

[Stefano Capaccio]

We use this library to create the Authorization headers for our SOAP requests.  However I was finding that if I want to add additional headers they were being overwritten by the library.

I was able to resolve this by changing oauth.signpost.basic.SoapURLCOnnectionRequestAdapter::setHeader() as follows:

FROM:

    @Override
    public void setHeader(String name, String value) {
        Map<String, List<String>> headers = new HashMap<>();
        headers.put(name, Collections.singletonList(value));
        this.connection.put(MessageContext.HTTP_REQUEST_HEADERS, headers);
    }

TO:

@Override
    public void setHeader(String name, String value) {
        // Ensure that any existing headers are not trashed, but retained
        Map<String, List<String>> headers = (Map<String, List<String>>) this.connection.get(MessageContext.HTTP_REQUEST_HEADERS);
        if (headers == null) {
            headers = new HashMap<String, List<String>>();
        }

        headers.put(name, Collections.singletonList(value));
        this.connection.put(MessageContext.HTTP_REQUEST_HEADERS, headers);
    }

This may help others, and maybe a better flow in the setHeader() function?

Additionally, the interface method oauth.signpost.http.HttpRequest::getAllHeaders() is typed incorrectly, the return value ought to be:

Map<String, List<String>>

rather than

Map<String, String>

(makes inserting headers back into the request impossible due to the underlying cast from String to List<String>

Thanks otherwise for a great simple library that does what it says on the tin!