`Yaml.Text(...)` fumbles without trailing whitespace

Describe the bug

Many editors either automatically remove trailing whitespace or have extensions that do so. Unfortunately, when trailing whitespace is removed from (say) Serialization/SerializationTests.cs, multiple tests fail with e.g.,

[xUnit.net 00:00:03.99]     ExampleFromSpecificationIsHandledCorrectly [FAIL]
  Failed ExampleFromSpecificationIsHandledCorrectly [1 ms]
  Error Message:
   System.ArgumentOutOfRangeException : startIndex cannot be larger than length of string. (Parameter 'startIndex')
  Stack Trace:
     at System.String.Substring(Int32 startIndex, Int32 length)
   at System.String.Substring(Int32 startIndex)
   at YamlDotNet.Test.Yaml.<>c__DisplayClass10_0.<Text>b__2(String l) in C:\dd\dnx\other\YamlDotNet\YamlDotNet.Test\Yaml.cs:line 111
   at System.Linq.Enumerable.SelectListIterator`2.ToList()
   at YamlDotNet.Test.Yaml.Text(String yamlText) in C:\dd\dnx\other\YamlDotNet\YamlDotNet.Test\Yaml.cs:line 110
   at YamlDotNet.Test.Serialization.SerializationTests.ExampleFromSpecificationIsHandledCorrectly() in C:\dd\dnx\other\YamlDotNet\YamlDotNet.Test\Serialization\SerializationTests.cs:line 1230

That particular test fails at the empty line near the top of the yamlText:

                obj:
                  - &CENTER { x: 1, y: 2 }
                  - &LEFT { x: 0, y: 2 }
                  - &BIG { r: 10 }
                  - &SMALL { r: 1 }

                # All the following maps are equal:

Fix is fairly simple. I'll put up a PR in a bit…

To Reproduce

Edit Serialization/SerializationTests.cs and remove all trailing whitespace, at least in the ExampleFromSpecificationIsHandledCorrectly() fact. Then, execute that test.

aaubry / YamlDotNet

`Yaml.Text(...)` fumbles without trailing whitespace #886