Add test of illegal replacement char

Author: sebbASF

src/main/java/org/apache/commons/io/FileSystem.java

@@ -211,7 +211,8 @@ public String toLegalFileName(final String candidate, final char replacement) {
         if (isIllegalFileNameChar(replacement)) {
             throw new IllegalArgumentException(
                     String.format("The replacement character '%s' cannot be one of the %s illegal characters: %s",
-                            replacement, name(), Arrays.toString(illegalFileNameChars)));
+                            // %s does not work properly with NUL
+                            replacement == '\0' ? "\\0" : replacement, name(), Arrays.toString(illegalFileNameChars)));
         }
         final String truncated = candidate.length() > maxFileNameLength ? candidate.substring(0, maxFileNameLength)
                 : candidate;

src/test/java/org/apache/commons/io/FileSystemTestCase.java

@@ -52,7 +52,7 @@ public void testToLegalFileNameWindows() {
         for (char i = '0'; i < '9'; i++) {
             Assert.assertEquals(i, fs.toLegalFileName(String.valueOf(i), replacement).charAt(0));
         }
-    }    
+    }
 
     @Test
     public void testIsLegalName() {
@@ -63,4 +63,15 @@ public void testIsLegalName() {
             Assert.assertTrue(fs.name(), fs.isLegalFileName("0")); // Assume simple name always legal
         }
     }
+
+    @Test
+    public void testReplacementWithNUL() {
+        for (FileSystem fs : FileSystem.values()) {
+            try {
+                fs.toLegalFileName("Test", '\0'); // Assume NUL is always illegal
+            } catch (IllegalArgumentException iae) {
+                Assert.assertTrue(iae.getMessage(), iae.getMessage().startsWith("The replacement character '\\0'"));
+            }
+        }
+    }
 }