Fix lowercase hex digit character references not being tokenized

Fixes shadowfacts/Tusker#459

Sources/HTMLStreamer/Tokenizer.swift

@@ -1555,12 +1555,12 @@ private extension Unicode.Scalar {
         case "7": 7
         case "8": 8
         case "9": 9
-        case "A": 0xA
-        case "B": 0xB
-        case "C": 0xC
-        case "D": 0xD
-        case "E": 0xE
-        case "F": 0xF
+        case "A", "a": 0xA
+        case "B", "b": 0xB
+        case "C", "c": 0xC
+        case "D", "d": 0xD
+        case "E", "e": 0xE
+        case "F", "f": 0xF
         default: nil
         }
     }

Tests/HTMLStreamerTests/TokenizerTests.swift

@@ -33,6 +33,8 @@ final class TokenizerTests: XCTestCase {
     func testNumericCharacterReference() {
         XCTAssertEqual(tokenize("!"), [.characterSequence("!")])
         XCTAssertEqual(tokenize("!"), [.characterSequence("!")])
+        XCTAssertEqual(tokenize("J"), [.characterSequence("J")])
+        XCTAssertEqual(tokenize("J"), [.characterSequence("J")])
     }
     
     func testStartTag() {