fixed problem with Utf-8 string

When string is utf-8 coded, strtolower breaks strings. mb_string should be used when available

src/Faker/Provider/Base.php

@@ -126,4 +126,24 @@ class Base
 	{
 		return static::lexify(static::numerify($string));
 	}
-}
+
+	/**
+	 * Converts string to lowercase.
+	 * Uses mb_string extension if available
+	 * @param string $string String that should be converted to lowercase
+	 * @return string
+	 */
+	public static function toLower($string) {
+		return extension_loaded('mbstring') ? mb_strtolower($string) : strtolower($string);
+	}
+
+	/**
+	 * Converts string to uppercase.
+	 * Uses mb_string extension if available
+	 * @param string $string String that should be converted to uppercase
+	 * @return string
+	 */
+	public static function toUpper($string) {
+		return extension_loaded('mbstring') ? mb_strtoupper($string) : strtoupper($string);
+	}
+}

src/Faker/Provider/Internet.php

@@ -70,7 +70,7 @@ class Internet extends \Faker\Provider\Base
 	public function userName()
 	{
 		$format = static::randomElement(static::$userNameFormats);
-		return strtolower(static::bothify($this->generator->parse($format)));
+		return static::toLower(static::bothify($this->generator->parse($format)));
 	}
 
 	/**
@@ -91,7 +91,7 @@ class Internet extends \Faker\Provider\Base
 		$company = $companyElements[0];
 		$company = preg_replace('/\W/', '', $company);
 
-		return strtolower($company);
+		return static::toLower($company);
 	}
 
 	/**