Avoid usage of constructor introduced in Android 2.3

The constructor `String(byte[] data, Charset charset)` has only been introduced in Android 2.3 while the constructor `String(byte[] data, String charsetName)` is available on *all* Android versions.

Source: http://developer.android.com/reference/java/lang/String.html

The README says `[...] Android, tested on version 2.2 and forward [...]` but the library will crash on Android 2.2 without this patch.