ClaimsPrincipalBuilder.cs

public class ClaimsPrincipalBuilder
{
    public static ClaimsPrincipal FromDictionary(IDictionary<string, string> source)
    {
        var claims = source.Select(pair => new Claim(pair.Key, pair.Value));
        var identity = new ClaimsIdentity(claims);
        return new ClaimsPrincipal(identity);
    }
}

UseHeaderClaims.cs

public static IApplicationBuilder UseHeaderClaims(this IApplicationBuilder app)
{
    return app.Use(next => async context =>
    {
        string[] claims;
        if (!context.Request.Headers.TryGetValue("Jwt-Claim", out claims))
        {
            context.Response.StatusCode = 401;
            return;
        }

        var dict = claims.Select(c => c.Split(Eq, 2))
            .ToDictionary(a => a[0], a => a[1]);

        context.User = ClaimsPrincipalBuilder.FromDictionary(dict);

        await next(context);
    });
}