Interface for savon-multipart for Savon2

The motivation behind this change is to provide the savon-multipart gem a hook
so that one can use it with savon. There is little desire (or much reason) to
fix this directly in Savon since very few people will ever use it. So long as
there is a mechanism for end users of savon-multipart to specify this, it should
be easy for Savon to refactor without needing to make changes for multipart
support.

The indended use case for this would be to either configure a global or local
multipart key to :true like so:

client = Savon::Client('http://service.example.com', :multipart => true)

# or for only a single call on a client
client.call(:some_method_returns_multipart, :multipart => true)