You need something like
foreach (JProperty prop in content.Properties())
{
if (prop.Name == Key)
{
Assert.That(prop.Value, Is.EqualTo(Value));
}
}
or
foreach (JProperty prop in content.Properties().Where(p => p.Name == Key))
{
Assert.That(prop.Value, Is.EqualTo(Value));
}
As an aside, these aren't proper unit tests. You should write a test for the class that handles your api requests instead. That lets you ensure the logic is valid and doesn't require the project to be running somewhere, you should be able to run your unit tests anywhere, they shouldn't have dependencies on databases, servers etc. For example the build server will want to run your unit tests but how can it when it needs all sorts of config to find your api?
By testing it from the client level like you are doing you are testing two things...one is that your API does what you want, the other is that the HttpGet function is working. The problem here is that a) your test tests multiple things, it should only test one b) you didn't write the HttpGet, it's not your code so why are you testing it?