Fix HexFormatter and add HexFormatter test.

src/Formatters/HexFormatter.php

@@ -30,8 +30,6 @@ class HexFormatter implements IFormatter
 
         if (Utils::isZeroPrefixed($value)) {
             return $value;
-        } elseif (Utils::isHex($value)) {
-            $value = '0x' . $value;
         } else {
             $value = Utils::toHex($value, true);
         }

test/unit/HexFormatterTest.php

@@ -0,0 +1,49 @@
+<?php
+
+namespace Test\Unit;
+
+use Test\TestCase;
+use Web3\Formatters\HexFormatter;
+
+class HexFormatterTest extends TestCase
+{
+    /**
+     * formatter
+     * 
+     * @var \Web3\Formatters\HexFormatter
+     */
+    protected $formatter;
+
+    /**
+     * setUp
+     * 
+     * @return void
+     */
+    public function setUp()
+    {
+        parent::setUp();
+        $this->formatter = new HexFormatter;
+    }
+
+    /**
+     * testFormat
+     * 
+     * @return void
+     */
+    public function testFormat()
+    {
+        $formatter = $this->formatter;
+
+        $hex = $formatter->format('ae');
+        $this->assertEquals($hex, '0x6165');
+
+        $hex = $formatter->format('0xabce');
+        $this->assertEquals($hex, '0xabce');
+
+        $hex = $formatter->format('123');
+        $this->assertEquals($hex, '0x7b');
+
+        $hex = $formatter->format(12);
+        $this->assertEquals($hex, '0xc');
+    }
+}