Error messages improvements

[faddiv]

Hy Kevin,

At last, I'm finished with my error messages improvements. :)
I also put several other improvements in the code. Here is the list:

• Improved error messages
• Documentation comments now visible.
• Merged the methods of ActionResultAssertions's parameterless methods with their counterparts.
• Extracted FileResultAssertions common method into base class which fixes some method's return type.
• Both PhysicalFileResultAssertions and VirtualFileResultAssertions have WithFileName. Also they have dedicated: WithPhysicalFile and WithVirtualFile.

Example for "Improved error messages":

[Fact]
public void Test1()
{
	var controller = new HomeController();

	var result = controller.Index();

	result.Should().BeJsonResult()
		.ValueAs<string>();
}

Expected ActionResult to be "JsonResult", but found "ViewResult" -> Expected result to be of type Microsoft.AspNetCore.Mvc.JsonResult but was Microsoft.AspNetCore.Mvc.ViewResult.

Expected "Value" to be of type '"String"' but was '"Int32"' -> Expected JsonResult.Value to be of type System.String but was System.Int32.

• Documentation comments now visible. Until now these wasn't published with the nuget package, but now there it is:

//
// Summary:
//     Asserts that the subject is an Microsoft.AspNetCore.Mvc.JsonResult.
//
// Parameters:
//   reason:
//     A formatted phrase as is supported by System.String.Format(System.String,System.Object[])
//     explaining why the assertion is needed. If the phrase does not start with the
//     word because, it is prepended automatically.
//
//   reasonArgs:
//     Zero or more objects to format using the placeholders in reason.
[CustomAssertion]
public JsonResultAssertions BeJsonResult(string reason = "", params object[] reasonArgs);

[faddiv]

@SoftwareWizard I would like to ask about one of your improvement: The WithAuthenticationProperties method, which is on several Assertions, checks the property by reference not by equivalence. Is this intentional? If yes then these error messages should be improved more.
Also you are welcome to check my work. Is these changes okay for you?

[faddiv]

Oh I almost forgot:
I also fixed the following bug:

• reason and reasonArgs wasn't included in every message.

[kevinkuszyk]

@SoftwareWizard do these changes look ok to you?

[SoftwareWizard]

Hi Kevin,
thanks for your improvements.
Looks good so far for me.

[kevinkuszyk]

@faddiv @SoftwareWizard thanks for this.

I have to admit, I just scanned through the changes, but nothing bad caught my eye.