SmallBigData - allowing messages longer than 32766 characters

Fixes work item #3040

Non .NET Framework 4 clients used the FormUrlEncodedContent class to escape messages sent by clients. This limited the length of the message to 32766 characters. For longer the client would fail with the invalid uri exception saying "The Uri string is too long". The messages are not actually Uris so we don't care about the length. The fix is to use a custom UrlEncoder we already have which is functionally equivalent to Uri.EscapeDataString() but will not throw if message is longer than 32766 characters.