INT-3199 Polishing

Only revert to RequestReplyExchanger if there are ambiguous
fallback methods - if the target object is an RRE, it will always
have a candidate fallback (exchange).

Add more RRE tests.

Make ParametersWrapper a public inner class - it's only used
within the context of a process() call. Making it public allows
SpEL to access its properties without reflection.