Adding reference to System.Net.WebSockets.Client in test library (#299)

Adding reference to System.Net.WebSockets.Client in test library.
Takes care of failing test added in #288

test/Microsoft.Azure.ServiceBus.UnitTests/Microsoft.Azure.ServiceBus.UnitTests.csproj

@@ -28,6 +28,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.0.0" />
+    <PackageReference Include="System.Net.WebSockets.Client" Version="4.3.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.2.0" />
     <PackageReference Include="Microsoft.NETCore.Platforms" Version="1.1.0" />
     <PackageReference Include="xunit" Version="2.2.0" />