added missing writer for Savon.http_adapter
fixed typo [ci skip]
first code and specs to support attributes

example Hashes contain attributes prefixed with _ (underscore).
the @-sign would be more natural for this, but it doesn't work
with the new ruby 1.9 hash syntax.

/cc savonrb/wasabi#27
fixed singularity for element ref's

we we're looking for the occurs attribute on the ref element,
not on the original element definition.