[cpp-ue4] Fix multi collection format has duplicated prefix

[nullbus]

Hello, I made another pull request that addresses different issue. Query parameter generation works very well except "multi" collection format. CollectionToUrlString_multi generates "key=value1&key=value2&..." like string and if used with query string, duplicated key string is appended like:

key=key=value1&key=value2

This commit separates "multi" type collection query string generation from other types.

@Kahncode

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[Kahncode]

Looks good, I don't think I have ever been able to test properly with this case.