IndexExistsAsync throws exception when index does not exist (not on par with Sync version)

[icanhasjonas]

var client = new ElasticClient( ... );
client.IndexExists( x => x.Index( "hello-world" ) ); // <--- OK
await client.IndexExistsAsync( x => x.Index( "hello-world" ) ); // <--- boom

[icanhasjonas]

I should add that this is the NuGet 1.0.0-beta and GetAsync<T>() vs Get<T>() and SourceAsync<T>() vs Source<T>() has the same issue

[gmarz]

Hey @icanhasjonas, this has already been fixed. See #654.