[mscorlib] Support OID names in RSACryptoServiceProvider.SignData()

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=13953

MSDN documents that RSACryptoServiceProvider.SignData()'s provider
paramter may contain an OID name:

http://msdn.microsoft.com/en-us/library/y2wf1b6k.aspx
> The halg parameter can accept a String, a HashAlgorithm, or a Type.
> The string value can be one of the following:
> * The object identifier (OID) friendly name of the hash algorithm to use,
>   either a name registered in the crypto config file or one in the
>   Crypto API OID table.
> * The OID value. The OID must be one recognized by the Crypto API.
> For example, you could use SignData(new byte[5], "1.3.14.3.2.26") or
> SignData(new byte[5], "sha1"), or SignData(new byte[5], "SHA1").

Add support for this OID names as the provider.