Added header value transformation method to MessageSupport

Author: ok2c

httpcore5-h2/src/main/java/org/apache/hc/core5/http2/priority/PriorityParser.java

@@ -50,9 +50,7 @@ public static PriorityValue parse(final Header header) {
         if (header == null) {
             return PriorityValue.defaults();
         }
-        final PriorityValue[] out = new PriorityValue[1];
-        MessageSupport.parseHeader(header, (seq, cur) -> out[0] = parse(seq, cur));
-        return out[0] != null ? out[0] : PriorityValue.defaults();
+        return MessageSupport.parserHeaderValue(header, PriorityParser::parse);
     }
 
     public static PriorityValue parse(final String headerValue) {

httpcore5/src/main/java/org/apache/hc/core5/http/message/MessageSupport.java

@@ -36,6 +36,7 @@
 import java.util.Set;
 import java.util.TreeSet;
 import java.util.function.BiConsumer;
+import java.util.function.BiFunction;
 import java.util.function.Consumer;
 import java.util.function.UnaryOperator;
 
@@ -202,6 +203,23 @@ public static void parseHeader(final Header header, final BiConsumer<CharSequenc
         }
     }
 
+    /**
+     * @since 5.5
+     */
+    public static <T> T parserHeaderValue(final Header header, final BiFunction<CharSequence, ParserCursor, T> transformation) {
+        Args.notNull(header, "Header");
+        if (header instanceof FormattedHeader) {
+            final CharArrayBuffer buf = ((FormattedHeader) header).getBuffer();
+            final ParserCursor cursor = new ParserCursor(0, buf.length());
+            cursor.updatePos(((FormattedHeader) header).getValuePos());
+            return transformation.apply(buf, cursor);
+        } else {
+            final String value = header.getValue();
+            final ParserCursor cursor = new ParserCursor(0, value.length());
+            return transformation.apply(value, cursor);
+        }
+    }
+
     /**
      * @since 5.4
      */