Use "@default" for currentSchema and currentCatalog (when it could be…

… null).

The problematic case that this change addresses is:

Imagine the connection is "fresh", and the connection-string defines something like schema=S1

So the current schema of the connection would be S1, but currentSchema is still null. So we prepare a statement for S1, but the cache key contains null.

If we then invoke setSchema(null) the connection is in schema-less state. Preparing the same statement would retrieve the one from above, which is prepared for S1.

---
The FIX for this case, use "@default" instead of null, such that the cache key isn't problematic for the above case.

ebean-datasource/src/main/java/io/ebean/datasource/pool/PooledConnection.java

@@ -124,6 +124,8 @@ final class PooledConnection extends ConnectionDelegator {
     this.originalCatalog = pool.catalog();
     this.initialisedSchema = originalSchema != null;
     this.initialisedCatalog = originalCatalog != null;
+    this.currentSchema = originalSchema != null ? originalSchema : "@default";
+    this.currentCatalog = originalCatalog != null ? originalCatalog : "@default";
     this.pstmtCache = new PstmtCache(pool.pstmtCacheSize());
     this.maxStackTrace = pool.maxStackTraceSize();
     this.creationTime = System.currentTimeMillis();