Updated fix for changed fields

[sveseli]

This PR updates condition for determining master field added in PR #82. A number of new tests have been added for the use case where PV request specifies individual fields.

[anjohnson]

Was this a necessary condition that you missed before?

Can you explain why here the master field's name is an empty string, but in #82 it was a single underscore "_"?

[sveseli]

Yes, I simply missed this when testing, and discovered the issue a few days ago. We indicate request for the top level structure (master field) in the request string by "_" as a convention that we decided a while ago when we discussed interface for the initial version of the data distributor plugin, but I do not think that "_" is actually set as the top level record name.

[anjohnson]

I don't claim to understand how the master field is used, but #82 did refer to it and pvCopy.cpp does look for it in the pvRequest. Please confirm there's no incompatibility between that and the code here (I don't need a detailed explanation, just for you to confirm they do both make sense).

[sveseli]

There should be no incompatibility, and I added tests to make sure the problem I saw was addressed.