Added multibyte string encoding consideration with tests

src/Filters/Capitalize.php

@@ -14,6 +14,6 @@ class Capitalize implements Filter
      */
     public function apply($value, $options = [])
     {
-        return is_string($value) ? ucwords(strtolower($value)) : $value;
+        return is_string($value) ? mb_convert_case(mb_strtolower($value, 'UTF-8'),  MB_CASE_TITLE) : $value;
     }
 }

src/Filters/Lowercase.php

@@ -14,6 +14,6 @@ class Lowercase implements Filter
      */
     public function apply($value, $options = [])
     {
-        return is_string($value) ? strtolower($value) : $value;
+        return is_string($value) ? mb_strtolower($value, 'UTF-8') : $value;
     }
 }

src/Filters/Uppercase.php

@@ -14,6 +14,6 @@ class Uppercase implements Filter
      */
     public function apply($value, $options = [])
     {
-        return is_string($value) ? strtoupper($value) : $value;
+        return is_string($value) ? mb_strtoupper($value) : $value;
     }
 }

tests/Filters/CapitalizeTest.php

@@ -24,4 +24,13 @@ public function it_capitalizes_strings()
         $result = $this->sanitize(['name' => ' jon snow 145'], ['name' => 'capitalize']);
         $this->assertEquals(' Jon Snow 145', $result['name']);
     }
+
+    /**
+     *  @test
+     */
+    public function it_capitalizes_special_characters()
+    {
+        $result = $this->sanitize(['name' => 'Τάχιστη αλώπηξ'], ['name' => 'capitalize']);
+        $this->assertEquals('Τάχιστη Αλώπηξ', $result['name']);
+    }
 }

tests/Filters/LowercaseTest.php

@@ -30,4 +30,19 @@ public function it_lowercases_strings()
         $data = $this->sanitize($data, $rules);
         $this->assertEquals('hello everybody', $data['name']);
     }
+
+    /**
+     *  @test
+     */
+    public function it_lowercases_special_characters_strings()
+    {
+        $data = [
+            'name' => 'Τάχιστη αλώπηξ',
+        ];
+        $rules = [
+            'name' => 'lowercase',
+        ];
+        $data = $this->sanitize($data, $rules);
+        $this->assertEquals('τάχιστη αλώπηξ', $data['name']);
+    }
 }

tests/Filters/UppercaseTest.php

@@ -30,4 +30,19 @@ public function it_uppercases_strings()
         $data = $this->sanitize($data, $rules);
         $this->assertEquals('HELLO EVERYBODY', $data['name']);
     }
+
+    /**
+     *  @test
+     */
+    public function it_uppercases_special_characters_strings()
+    {
+        $data = [
+            'name' => 'Τάχιστη αλώπηξ',
+        ];
+        $rules = [
+            'name' => 'uppercase',
+        ];
+        $data = $this->sanitize($data, $rules);
+        $this->assertEquals('ΤΆΧΙΣΤΗ ΑΛΏΠΗΞ', $data['name']);
+    }
 }