[cpp-ue4] add support for Content-Type containing charset

[Falanwe]

Change to the api template for cpp-ue4 to handle a composite Content-Type (for instance application/json; charset=utf-8) by comparing the start of the string instead of looking for a perfect match.

Sadly, there are no test config for cpp-ue4 (yet?). I tested the change directly inside the UE4 project we have in which we are using openapi-generator (as it's a proprietary game I cannot share it obviously, but the resulting code did compile well).

@ravinikam @etherealjoy @MartinDelille @muttleyxd @Kahncode

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[wing328]

@Falanwe thanks for the PR.

Later we may want to use a regular expression to handle more edge cases, e.g.

openapi-generator/samples/client/petstore/csharp/OpenAPIClient/src/Org.OpenAPITools/Client/ApiClient.cs

Line 364 in 5693cb9

var jsonRegex = new Regex("(?i)^(application/json|[^;/ \t]+/[^;/ \t]+[+]json)[ \t]*(;.*)?$");

We can go with your suggested fix for the time being.

[Falanwe]

I'm not really familiar with using regexes in UE4 (I'm more of a C# programmer usually, but I think it would be a good idea to use them indeed). I'll probably create a new pull request with that when I have a little time, getting inspiration from C#'s regex (hopefully the regex syntax is the same between the two, at least for this specific regex).