encoding: make Block convertible to span

Change-Id: Idb501467d1c56b95e7fdb76acda63b919fe98f85
diff --git a/tests/unit/meta-info.t.cpp b/tests/unit/meta-info.t.cpp
index 56ad239..af8416a 100644
--- a/tests/unit/meta-info.t.cpp
+++ b/tests/unit/meta-info.t.cpp
@@ -139,9 +139,7 @@
 
     block = info2.findAppMetaInfo(tlvType);
     BOOST_REQUIRE(block != nullptr);
-
-    std::string s3(reinterpret_cast<const char*>(block->value()), block->value_size());
-    BOOST_CHECK_EQUAL(s3, ss[i]);
+    BOOST_CHECK_EQUAL(readString(*block), ss[i]);
   }
 }