fix csharp generated code for string default value

[vicobill]

when generated C# code for table like :

table Monster{
    //...
    name:string  = "Default Monster Name",
   // ...
}

there is no way to save default string value before, after this PR, string has default value, some-like int default value.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.