OGR connection string shouldn't have trailing slash

[traianstanev]

Here is the description from Kenneth:

I have built an editor for the OGR provider in my MapStudio OS project. I noticed that the OGR provider does not accept the %MG_DATA_FILE_PATH% parameter but does accept another path. After digging around, I found out that OGR rejects directories as datasources, if they have a trailing slash. This prevents using the entire repository storage as a datasource, so I made a small patch that removes a trailing slash, if any.

There should be no compatibility issuses, as none of the other types of connectionstrings are likely to end with a backslash.

-- Regards, Kenneth, GEOGRAF A/S