Server side handling of @fromHeader
Handle @fromHeader client-side

- Add a string[string] headers__ local variable to the generated function;
- If there's one and only one WebParamAttribute with the name of the parameter we're currently processing (as parameter gets processed in order), we add it to the AA, the key being the 'field' in WebParamAttribute;
- headers__ gets later passed to request() which will add all it's field to the headers;