Enable send-receive round trip

Currently, the handling of process variables in "send to Camel" and "receive from Camel" is not symmetric.

Receive converts a message body of type Map to process variables, but Send does not produce a message body of this type, sending the raw value of a single process variable instead.

I've changed sendTo() to wrap the included process variable into a single-item hash map (allowing for future changes introducing overloaded methods with 2 or 3 variable names or using a separator character like 'var1,var2,var3').

With this change, a Camel receive task is able to consume a message sent from a service task via Camel.