[csharp] Options: useCollection, returnICollection

This change is inline with Microsoft's recommended guidelines for
collects
(https://msdn.microsoft.com/en-us/library/dn169389(v=vs.110).aspx).

Added generator options for csharp to:

* useCollection: Deserialize responses into and return Collection<T>
* returnICollection: For List<T> or Collection<T>, return ICollection<T>
  instead of the concrete type

As a consequence of useCollection, method imputs will also change to
Collection<T>.

modules/swagger-codegen/src/test/java/io/swagger/codegen/options/CSharpClientOptionsProvider.java

@@ -26,6 +26,8 @@ public class CSharpClientOptionsProvider implements OptionsProvider {
                 .put(CodegenConstants.OPTIONAL_ASSEMBLY_INFO, "true")
                 .put(CodegenConstants.USE_DATETIME_OFFSET, "true")
                 .put(CodegenConstants.SOURCE_FOLDER, SOURCE_FOLDER_VALUE)
+                .put(CodegenConstants.USE_COLLECTION, "false")
+                .put(CodegenConstants.RETURN_ICOLLECTION, "false")
                 .build();
     }