Test `ext/mbstring` availability globally.

First, in the constructor, we tested whether the `ext/mbstring`
extension was available or not. This check was made each time the
constructor run. This is useless and we can optimize it. Second, some
static methods were using `ext/mbstring`, so this check was not made and
this situation could lead to unexpected errors.

This patch introduces and testes the `checkMbString` static method. As
its name suggests, it checks whether the `ext/mbstring` extension is
available or not.

We finally use this method globally, after the class declaration, and
throw an exception if the extension is not available.

1 files changed, 11 insertions, 2 deletions

diff --git a/Test/Unit/Ustring.php b/Test/Unit/Ustring.php
index 0443892..a03db3c 100644
--- a/Test/Unit/Ustring.php
+++ b/Test/Unit/Ustring.php
@@ -36,8 +36,8 @@
 
 namespace Hoa\Ustring\Test\Unit;
 
-use Hoa\Ustring as LUT;
 use Hoa\Test;
+use Hoa\Ustring as LUT;
 
 /**
  * Class \Hoa\Ustring\Test\Unit\Ustring.
@@ -49,7 +49,16 @@ use Hoa\Test;
  */
 class Ustring extends Test\Unit\Suite
 {
-    public function case_no_mbstring()
+    public function case_check_mbstring()
+    {
+        $this
+            ->given($this->function->function_exists = true)
+            ->then
+                ->boolean(LUT::checkMbString())
+                    ->isTrue();
+    }
+
+    public function case_check_no_mbstring()
     {
         $this
             ->given(