Fix Presentation format (#33)

Author: AdrianSoundy

nanoFramework.Device.Bluetooth/GenericAttributeProfile/GattLocalCharacteristic.cs

@@ -98,7 +98,7 @@ internal GattLocalCharacteristic(Guid characteristicUuid, GattLocalCharacteristi
                 DataWriter dr = new DataWriter();
 
                 dr.WriteByte(gpf.FormatType);
-                dr.WriteInt32(gpf.Exponent);
+                dr.WriteByte((byte)gpf.Exponent);
                 dr.WriteUInt16(gpf.Unit);
                 dr.WriteByte(gpf.Namespace);
                 dr.WriteUInt16(gpf.Description);