Fix/csharp reserved headers and file parameter not serialising correctly #361

Job	Run time
Build .Net Framework projects (samples/client/petstore/csharp/generichost/net4.7/FormModels)	1m 35s
Build .Net Framework projects (samples/client/petstore/csharp/generichost/net4.8/UseDateTimeForDate)	1m 4s
Build .Net Framework projects (samples/client/petstore/csharp/generichost/net4.8/AnyOf)	1m 6s
Build .Net Framework projects (samples/client/petstore/csharp/generichost/net4.7/UseDateTimeForDate)	1m 32s
Build .Net Framework projects (samples/client/petstore/csharp/generichost/net4.7/Petstore)	1m 1s
Build .Net Framework projects (samples/client/petstore/csharp/generichost/net4.7/AllOf)	2m 13s
Build .Net Framework projects (samples/client/petstore/csharp/generichost/net4.8/AnyOfNoCompare)	1m 17s
Build .Net Framework projects (samples/client/petstore/csharp/generichost/net4.8/AllOf)	1m 34s
Build .Net Framework projects (samples/client/petstore/csharp/generichost/net4.8/FormModels)	1m 3s
Build .Net Framework projects (samples/client/petstore/csharp/generichost/net4.7/AnyOf)	1m 12s
Build .Net Framework projects (samples/client/petstore/csharp/generichost/net4.7/AnyOfNoCompare)	1m 5s
Build .Net Framework projects (samples/client/petstore/csharp/generichost/net4.8/Petstore)	5m 28s
Build .Net Framework projects (samples/client/petstore/csharp/generichost/net4.7/OneOf)	1m 11s
Build .Net Framework projects (samples/client/petstore/csharp/generichost/net4.8/OneOf)	1m 8s
	22m 29s

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