makes some reject helpers public

[cbusbey]

#121

cc @mgatny

[bhaan]

Do you think it would be sensible to also expose the rejectReason enum values?

I'm in the process of testing some error scenarios, and it seems logical to confirm the reject reason is what I expect it to be. The MessageRejectError interface provides an exposed RejectReason() int function, but the hidden enum values prevent me from making these assertions in a type safe way.

[cbusbey]

Possibly... The reject reasons are not comprehensive though. The enum package has all of them

• session rejects
• business rejects

Caveat being that these are strings.