CSharp GoTo is off by one column and line

Omnisharp lines and columns are 1-based, where YCM/vim are 0-based. This is already [correctly adjusted for in creating the request](https://github.com/Valloric/YouCompleteMe/blob/master/python/ycm/completers/cs/cs_completer.py#L218), but we need to adjust the line and column while processing the response as well.

I have already signed the CLA.