Ensure cast correctly included for OpPlus compilation

When the plus operator is used between strings in a SpEL
expression and that expression is compiled, it is
important to include a cast if computation of any of
the operands isn't obviously leaving strings on the
stack. Likewise if the stack contents are known to
be strings, a cast should not be included.

Issue: SPR-12426
(cherry picked from commit 58756b0)

spring-expression/src/main/java/org/springframework/expression/spel/ast/OpPlus.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -206,7 +206,7 @@ public class OpPlus extends Operator {
 		else {
 			cf.enterCompilationScope();
 			operand.generateCode(mv,cf);
-			if (cf.lastDescriptor() != "Ljava/lang/String") {
+			if (!"Ljava/lang/String".equals(cf.lastDescriptor())) {
 				mv.visitTypeInsn(CHECKCAST, "java/lang/String");
 			}
 			cf.exitCompilationScope();