Add test for version.json version with invalid value

Repro for #251

Author: AArnott

src/NerdBank.GitVersioning.Tests/VersionOracleTests.cs

@@ -289,4 +289,15 @@ public void VersionJsonWithoutVersion()
         Assert.Equal(0, oracle.Version.Major);
         Assert.Equal(0, oracle.Version.Minor);
     }
+
+    [Fact]
+    public void VersionJsonWithSingleIntegerForVersion()
+    {
+        File.WriteAllText(Path.Combine(this.RepoPath, VersionFile.JsonFileName), @"{""version"":""3""}");
+        this.InitializeSourceControl();
+        var ex = Assert.Throws<FormatException>(() => VersionOracle.Create(this.RepoPath));
+        Assert.Contains(this.Repo.Head.Commits.First().Sha, ex.Message);
+        Assert.Contains("\"3\"", ex.InnerException.Message);
+        this.Logger.WriteLine(ex.ToString());
+    }
 }