Fix SpEL support for quotes within String literals

Prior to this commit, there were two bugs in the support for quotes
within String literals in SpEL expressions.

- Two double quotes ("") or two single quotes ('') were always replaced
  with one double quote or one single quote, respectively, regardless
  of which quote character was used to enclose the original String
  literal. This resulted in the loss of one of the double quotes when
  the String literal was enclosed in single quotes, and vice versa. For
  example, 'x "" y' became 'x " y'.

- A single quote which was properly escaped in a String literal
  enclosed within single quotes was not escaped in the AST string
  representation of the expression. For example, 'x '' y' became 'x ' y'.

This commit fixes both of these related issues in StringLiteral and
overhauls the structure of ParsingTests.

Closes gh-29604
Closes gh-28356

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -26,6 +26,7 @@ import org.springframework.util.StringUtils;
  *
  * @author Andy Clement
  * @author Juergen Hoeller
+ * @author Sam Brannen
  * @since 3.0
  */
 public class StringLiteral extends Literal {
@@ -36,9 +37,19 @@ public class StringLiteral extends Literal {
 	public StringLiteral(String payload, int startPos, int endPos, String value) {
 		super(payload, startPos, endPos);
 
+		// The original enclosing quote character for the string literal: ' or ".
+		char quoteCharacter = value.charAt(0);
+
+		// Remove enclosing quotes
 		String valueWithinQuotes = value.substring(1, value.length() - 1);
-		valueWithinQuotes = StringUtils.replace(valueWithinQuotes, "''", "'");
-		valueWithinQuotes = StringUtils.replace(valueWithinQuotes, "\"\"", "\"");
+
+		// Replace escaped internal quote characters
+		if (quoteCharacter == '\'') {
+			valueWithinQuotes = StringUtils.replace(valueWithinQuotes, "''", "'");
+		}
+		else {
+			valueWithinQuotes = StringUtils.replace(valueWithinQuotes, "\"\"", "\"");
+		}
 
 		this.value = new TypedValue(valueWithinQuotes);
 		this.exitTypeDescriptor = "Ljava/lang/String";
@@ -52,7 +63,9 @@ public class StringLiteral extends Literal {
 
 	@Override
 	public String toString() {
-		return "'" + getLiteralValue().getValue() + "'";
+		String ast = String.valueOf(getLiteralValue().getValue());
+		ast = StringUtils.replace(ast, "'", "''");
+		return "'" + ast + "'";
 	}
 
 	@Override