"help" crashes when mixing UsageAttribute with other Attributes

**Issue by [LarsWeuste](https://github.com/LarsWeuste)**
_Friday May 19, 2017 at 10:47 GMT_
_Originally opened as https://github.com/gsscoder/commandline/issues/443_

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When using the "Usage" attribute together with a 2nd one (e.g. ExcludeFromCoverage) like this:

[Usage]
[ExcludeFromCodeCoverage]
public static IEnumerable Examples ...

In this situation displaying "help" will cause an exception.
The cause for that is likely fixed by this change:

src/CommandLine/Core/ReflectionExtensions.cs: L45

public static Maybe> GetUsageData(this Type type)
{
return
(from pi in type.FlattenHierarchy().SelectMany(x => x.GetTypeInfo().GetProperties())
let attrs = pi.GetCustomAttributes(true)
where attrs.OfType().Any()
select Tuple.Create(pi, (UsageAttribute)attrs.OfType().First()))
.SingleOrDefault()
.ToMaybe();
}

從 src/CommandLine/Core/ReflectionExtensions.cs 開始，特別查看第 45 行附近的 GetUsageData。在同一個 property 上使用 Usage 和 ExcludeFromCodeCoverage 屬性重現 help 命令，然後確認 help 顯示時不會發生例外，且相關行為仍由現有測試涵蓋。

由索引模型根據 Issue 內容生成。