merge `respond` tests with other helper tests
mock --> double
update libertree-client
replace `let` definition of helper
build_stanza expects an XML string
remove similar test
use client lib for requester
remove comment about XML representation

The XML representation is handled by the client lib.
undo unnecessary wrapping

Each reference was wrapped in a hash from 'reference' to the actual
reference data to make it trivial to convert the hash to XML.  Actually,
though, it made it more difficult to deal with references as the
wrapping had to be undone by the receiver.  That's just ugly.
support command tags without children
rewrite xml_to_hash

- easier to understand
- better tests
- merge array of hashes to hash
fix post responder with references
fix comment responder with references
adapt forest responder to new client XML format
fix: reduce to array, not hash