Fix unchecked warning introduced in 5559ba1

Factor the work of lax() into a static method on AdjustingJAXPParser
but push the instance methods down to the concrete subclasses, which
can return 'this' with the expected type and no unchecked warning.

pljava/src/main/java/org/postgresql/pljava/jdbc/SQLXMLImpl.java

@@ -4203,16 +4203,20 @@ protected Exception exceptions()
 			return e;
 		}
 
-		@Override
-		public T lax(boolean discard)
+		/**
+		 * Common factor of subclass {@link #lax() lax()} instance methods.
+		 *<p>
+		 * The work is done here, but the instance methods are implemented
+		 * per-subclass to avoid unchecked casting of 'this'.
+		 */
+		protected static void lax(AdjustingJAXPParser o, boolean discard)
 		{
-			if ( null != m_quiet )
+			if ( null != o.m_quiet )
 			{
 				if ( ! discard )
-					addSignaling(m_quiet);
-				m_quiet = null;
+					o.addSignaling(o.m_quiet);
+				o.m_quiet = null;
 			}
-			return (T)this;
 		}
 
 		@Override
@@ -5093,6 +5097,13 @@ public SAXSource get() throws SQLException
 			return ss;
 		}
 
+		@Override
+		public AdjustingSAXSource lax(boolean discard)
+		{
+			lax(this, discard);
+			return this;
+		}
+
 		@Override
 		public AdjustingSAXSource defaults()
 		{
@@ -5215,6 +5226,13 @@ public SAXResult get() throws SQLException
 			return sr;
 		}
 
+		@Override
+		public AdjustingSAXResult lax(boolean discard)
+		{
+			lax(this, discard);
+			return this;
+		}
+
 		@Override
 		public AdjustingSAXResult xIncludeAware(boolean v)
 		{
@@ -5324,6 +5342,13 @@ public StAXSource get() throws SQLException
 			return ss;
 		}
 
+		@Override
+		public AdjustingStAXSource lax(boolean discard)
+		{
+			lax(this, discard);
+			return this;
+		}
+
 		@Override
 		public AdjustingStAXSource allowDTD(boolean v) {
 			return setFirstSupportedFeature( v, XMLInputFactory.SUPPORT_DTD);
@@ -5457,6 +5482,13 @@ public DOMSource get() throws SQLException
 			return ds;
 		}
 
+		@Override
+		public AdjustingDOMSource lax(boolean discard)
+		{
+			lax(this, discard);
+			return this;
+		}
+
 		@Override
 		public AdjustingDOMSource xIncludeAware(boolean v)
 		{