Before the change the gem crashed when converting a malformed string to date/timestamp
object. Right now, when there is an exception caught during mapping soap parameters to
Ruby's objects a proper error message is sent.

Send an error message when any of the soap parameters are missing. Before the change,
the gem would not send an error message, because while mapping parameters to Ruby's
objects only parameters existing on xml_data map are processed by Param#load method,
which checks for missing parameters.

Changed http error code from 500 to 422

Bumped version to 0.6.1