Make volatile

HasStateChanged will be accessed from multiple threads and therefore must be made volatile.
<a href='#crh-start'></a><a href='#crh-data-%7B%22processed%22%3A%20%5B%22https%3A//github.com/AzureAD/azure-activedirectory-library-for-dotnet/pull/124%23issuecomment-62084604%22%2C%20%22https%3A//github.com/AzureAD/azure-activedirectory-library-for-dotnet/pull/124%23issuecomment-67431360%22%2C%20%22https%3A//github.com/AzureAD/azure-activedirectory-library-for-dotnet/pull/124%23issuecomment-67431371%22%2C%20%22https%3A//github.com/AzureAD/azure-activedirectory-library-for-dotnet/pull/124%23issuecomment-67431398%22%5D%2C%20%22comments%22%3A%20%7B%22General%20Comment%22%3A%20%7B%22html_url%22%3A%20%22https%3A//github.com/AzureAD/azure-activedirectory-library-for-dotnet/pull/124%23issuecomment-62084604%22%2C%20%22comments%22%3A%20%5B%7B%22body%22%3A%20%22Please%20send%20your%20pull%20request%20to%20the%20%27servicing%27%20branch%20and%20we%20will%20take%20it.%20%27master%27%20branch%20is%20locked.%20Thanks.%22%2C%20%22created_at%22%3A%20%222014-11-07T01%3A55%3A49Z%22%2C%20%22user%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/3790715%3Fv%3D3%22%2C%20%22html_url%22%3A%20%22https%3A//github.com/afshins%22%7D%7D%2C%20%7B%22body%22%3A%20%22%3A%2B1%3A%22%2C%20%22created_at%22%3A%20%222014-12-18T02%3A10%3A16Z%22%2C%20%22user%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/3790715%3Fv%3D3%22%2C%20%22html_url%22%3A%20%22https%3A//github.com/afshins%22%7D%7D%2C%20%7B%22body%22%3A%20%22%3Ashipit%3A%22%2C%20%22created_at%22%3A%20%222014-12-18T02%3A10%3A25Z%22%2C%20%22user%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/3790715%3Fv%3D3%22%2C%20%22html_url%22%3A%20%22https%3A//github.com/afshins%22%7D%7D%2C%20%7B%22body%22%3A%20%22%3Ashipit%3A%22%2C%20%22created_at%22%3A%20%222014-12-18T02%3A10%3A42Z%22%2C%20%22user%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/3790715%3Fv%3D3%22%2C%20%22html_url%22%3A%20%22https%3A//github.com/afshins%22%7D%7D%5D%2C%20%22title%22%3A%20%22General%20Comment%22%7D%7D%2C%20%22approved%22%3A%20%7B%22https%3A//github.com/afshins%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/3790715%3Fv%3D3%22%7D%7D%7D'></a>
<a href='https://www.codereviewhub.com/'><img src='http://www.codereviewhub.com/site/github-bar.png' height=40></a>

<img src='http://www.codereviewhub.com/site/github-approved-avatar.png'><a href='https://github.com/afshins'><img src='https://avatars.githubusercontent.com/u/3790715?v=3' width=34 height=34></a>

- [x] <a href='#crh-comment-General Comment'></a> <img src='http://www.codereviewhub.com/site/github-completed.png' height=16 width=60>&nbsp;<b><a href='https://github.com/AzureAD/azure-activedirectory-library-for-dotnet/pull/124#issuecomment-62084604'>General Comment</a></b>
- <a href='https://github.com/afshins'><img border=0 src='https://avatars.githubusercontent.com/u/3790715?v=3' height=16 width=16'></a> Please send your pull request to the 'servicing' branch and we will take it. 'master' branch is locked. Thanks.


<a href='https://www.codereviewhub.com/AzureAD/azure-activedirectory-library-for-dotnet/pull/124?mark_as_completed=1'><img src='http://www.codereviewhub.com/site/github-mark-as-completed.png' height=26></a>&nbsp;<a href='https://www.codereviewhub.com/AzureAD/azure-activedirectory-library-for-dotnet/pull/124?approve=1'><img src='http://www.codereviewhub.com/site/github-approve.png' height=26></a>&nbsp;<a href='https://www.codereviewhub.com/AzureAD/azure-activedirectory-library-for-dotnet/pull/124?approve=0'><img src='http://www.codereviewhub.com/site/github-undo-approve.png' height=26></a>&nbsp;<a href='https://github.com/AzureAD/azure-activedirectory-library-for-dotnet/pull/124'><img src='http://www.codereviewhub.com/site/github-refresh.png' height=26></a>
<a href='#crh-end'></a>