`MarshalJSON` would not include the extra attributes because it iterated
over the target map rather than the source map.
Also fixes JSON unmarshaling for SSH and secrets. The intention was to
unmarshal into the struct, but `UnmarshalText` takes priority over the
default struct unmarshaling so it didn't work as intended.
Tests have been added for all marshaling and unmarshaling methods.
Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>
