Fix SimpleApiClient.IsEnterprise().

[grokys]

Previous unless you had called GetRepository on a SimpleApiClient instance, IsEnterprise() simply returned false without checking. Make sure we actually perform the check.

Fixes #1473

[shana]

Looks good. To note,

VisualStudio/src/GitHub.App/ViewModels/GitHubPane/GitHubPaneViewModel.cs

Line 371 in bf8f594

if ((isDotCom || isEnterprise) && await IsValidRepository(client))

is the "real" culprit (IsEnterprise being called before GetRepository, but since IsEnterprise doesn't require the repository object to be filled, the order dependency is not needed. WikiProbe, however, does require the repository object to be filled out, so we should audit it to make sure the api is sane.