Add ability to pass default_action (if HTTP_SOAPACTION undefined)

Hello,

I have read #24 and implemented idea described in [comment](https://github.com/inossidabile/wash_out/issues/24#issuecomment-4974502).

I can't pass HTTP_SOAPACTION because I am not owner of code. So, need to have solution to prevent this problem on my side. 

Thanks

P.S.
    Workaround with Rack middleware looks like guerrilla patch and I don' want to use it in my project.