Fix JNIEnv.FindClass

I was seeing this issue with FindClass:

```
May 22 15:57:43 Brunos-iPad arcadians[10034] <Warning>: Unhandled managed exception: CodePage 437 not supported (System.NotSupportedException)
          at System.Text.Encoding.GetEncoding (Int32 codepage) [0x00000] in <filename unknown>:0 
          at IKVM.Runtime.JNIEnv.StringFromOEM (System.Byte* psz) [0x00000] in <filename unknown>:0 
          at IKVM.Runtime.JNIEnv.FindClass (IKVM.Runtime.JNIEnv* pEnv, System.Byte* pszName) [0x00000] in <filename unknown>:0 
```

Not sure if assuming ascii is the right solution, but it fixes things for me.