Colon should not be encoded in all cases

- https://github.com/package-url/purl-spec/blob/0c3bc118ac5c001e067ba42fba8501405514f1a9/docs/standard/characters-and-encoding.md

Signed-off-by: Keshav Priyadarshi <[email protected]>

Author: keshav-space

src/main/java/com/github/packageurl/internal/StringUtil.java

@@ -22,11 +22,12 @@
 package com.github.packageurl.internal;
 
 import static java.lang.Byte.toUnsignedInt;
-
-import com.github.packageurl.ValidationException;
 import java.nio.charset.StandardCharsets;
+
 import org.jspecify.annotations.NonNull;
 
+import com.github.packageurl.ValidationException;
+
 /**
  * String utility for validation and encoding.
  *
@@ -52,6 +53,13 @@ public final class StringUtil {
         UNRESERVED_CHARS['.'] = true;
         UNRESERVED_CHARS['_'] = true;
         UNRESERVED_CHARS['~'] = true;
+
+        /*
+        According to purl-spec https://github.com/package-url/purl-spec/blob/0c3bc118ac5c001e067ba42fba8501405514f1a9/docs/standard/characters-and-encoding.md
+        > The following characters must not be percent-encoded:
+        > - the colon ':', whether used as a Separator Character or otherwise 
+         */
+        UNRESERVED_CHARS[':'] = true;
     }
 
     private StringUtil() {

src/test/resources/test-suite-data.json

@@ -86,7 +86,7 @@
   {
     "description": "docker uses qualifiers and hash image id as versions",
     "purl": "pkg:docker/customer/dockerimage@sha256:244fd47e07d1004f0aed9c?repository_url=gcr.io",
-    "canonical_purl": "pkg:docker/customer/dockerimage@sha256%3A244fd47e07d1004f0aed9c?repository_url=gcr.io",
+    "canonical_purl": "pkg:docker/customer/dockerimage@sha256:244fd47e07d1004f0aed9c?repository_url=gcr.io",
     "type": "docker",
     "namespace": "customer",
     "name": "dockerimage",