Fix decode in render_union in java

[bartekn]

My 1yo commit I forgot to merge. It fixes decode method in Java (discriminant value was not read). I used this to generate Java classes when XDR changed last year so they are OK.

Example, Memo class diff:

diff --git a/src/main/java/org/stellar/sdk/xdr/Memo.java b/src/main/java/org/stellar/sdk/xdr/Memo.java
index e04bf25..d66c5d2 100644
--- a/src/main/java/org/stellar/sdk/xdr/Memo.java
+++ b/src/main/java/org/stellar/sdk/xdr/Memo.java
@@ -80,8 +80,10 @@ public static void encode(XdrDataOutputStream stream, Memo encodedMemo) throws I
   }
   }
   public static Memo decode(XdrDataInputStream stream) throws IOException {
-    Memo decodedMemo = new Memo();
-    switch (decodedMemo.getDiscriminant()) {
+  Memo decodedMemo = new Memo();
+  MemoType discriminant = MemoType.decode(stream);
+  decodedMemo.setDiscriminant(discriminant);
+  switch (decodedMemo.getDiscriminant()) {
   case MEMO_NONE:
   break;
   case MEMO_TEXT: