Rewrite xUnit tests to remove warnings

[andmos]

After upgrading xUnit package, a couple of warnings shows up. The tests should be rewritten anyways, since some of the assert-usage is pointless in the first place.

TestBikeShareClient.cs(246,13): warning xUnit2002: Do not use Assert.NotNull() on value type 'DateTime'. [/app/TestBikeshareClient/TestBikeshareClient.csproj]
TestBikeShareClient.cs(258,13): warning xUnit2002: Do not use Assert.NotNull() on value type 'DateTime'. [/app/TestBikeshareClient/TestBikeshareClient.csproj]
TestBikeShareClient.cs(261,13): warning xUnit2002: Do not use Assert.NotNull() on value type 'int'. [/app/TestBikeshareClient/TestBikeshareClient.csproj]
TestBikeShareClient.cs(263,13): warning xUnit2002: Do not use Assert.NotNull() on value type 'int'. [/app/TestBikeshareClient/TestBikeshareClient.csproj]
TestBikeShareClient.cs(265,13): warning xUnit2002: Do not use Assert.NotNull() on value type 'int'. [/app/TestBikeshareClient/TestBikeshareClient.csproj]
TestBikeShareClient.cs(221,13): warning xUnit2012: Do not use Enumerable.Any() to check if a value exists in a collection. [/app/TestBikeshareClient/TestBikeshareClient.csproj]
TestBikeShareClient.cs(235,13): warning xUnit2012: Do not use Enumerable.Any() to check if a value exists in a collection. [/app/TestBikeshareClient/TestBikeshareClient.csproj]

[andmos]

Closed in #68