Add the ability to have safe params

[cheerfulstoic]

The issue I'm trying to fix: we're sending Base64-encoded files in some SOAP requests and seeing overhead of ~200ms per request that we'd like to avoid. After some investigation I found that it's possible to skip the escaping of values in XmlBuilder.

This adds the ability for users of soap to specify a {:__safe, _} value for a param. This will pass a {:safe, _} tuple down into XmlBuilder where we can avoid escaping strings that don't need to be escaped. I chose {:__safe, _} instead of {:safe, _} for soap because somebody might want to have a <safe> tag in their XML. Happy to talk that through.