Createable/Updateable attributes don't work on properties with a custom JsonProperty name

[avesse]

Example:

[Createable(false), Updateable(false)]
[JsonProperty("Salesforce_Name__c")]
public string DotnetName { get; set; }

The CreateableContractResolver and UpdateableContractResolver will not work in this case, since it's comparing the wrong property names. This can be fixed by replacing

var propInfo = type.GetRuntimeProperty(property.PropertyName);

with

var propInfo = type.GetRuntimeProperty(property.UnderlyingName);

in those two classes.