Support Mocking in OData Client.

[ElizabethOkerio]

Issues

This pull request fixes issue #1250.

Description

The DataServiceContext, DataServiceQuery, DataServiceQueryOfT and DataServiceQuerySingleOfT classes cannot be mocked as they are. In this PR, I have made these classes' public methods and properties virtual to support mocking.
Virtual methods can be mocked using any Mock Framework. Example, using the Moq Framework
You can mock the AddObject() method in the Microsoft.OData.Client.DataServiceContext class like below:

using Moq; 
using Xunit
namespace SampleMock
{
   /// How to mock the DataServiceContext class methods using moq framework example. 
    public class DataServiceContextMockTests 
    { 
        [Fact] 
        public void VerifyIfDataServiceContextAddObjectMethodIsMockable() 
        { 
            string str = "user"; 
            Mock mockObject = new Mock(); 
            mockObject.CallBase = true; 
            mockObject.Setup(x => x.AddObject(It.IsAny(), It.IsAny())).Callback(() => { str = "Saved Successfully"; }); 
            Users users = new Users(); 
            string result = users.SaveUser(mockObject.Object); 
            Assert.Equal(str, result); 
        }  
        /// A sample implementation class with methods calling the DataserviceContext.cs methods 
        public class Users 
        { 
            /// A simple method to save a user's details  
            /// A concrete class in the Microsoft.OData.Client library with implemented methods that this method calls.         
            public string SaveUser(DataServiceContext dataServiceContext) 
            { 
                string str = "user"; 
                object obj = null; 
                dataServiceContext.AddObject(str, obj); 
                return "Saved Successfully"; 
            } 
        } 
    }

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[paulodero]

Looks good but a few minor issues to be addressed.

[g2mula]

lgtm