Merging iniData models to support loading of multiple ini files into one structure

Ini files (iniData) can be merged by overwriting:

iniData a = ...
iniData b = ...
a.Merge(b); // merges content of b by overwriting into a

I reused an existing test file that there is no need to change a csproj.

Original PR: https://github.com/rickyah/ini-parser/pull/46