encoding: make Block convertible to span

Change-Id: Idb501467d1c56b95e7fdb76acda63b919fe98f85
diff --git a/tests/unit/mgmt/dispatcher.t.cpp b/tests/unit/mgmt/dispatcher.t.cpp
index 30b5853..a4897a9 100644
--- a/tests/unit/mgmt/dispatcher.t.cpp
+++ b/tests/unit/mgmt/dispatcher.t.cpp
@@ -398,10 +398,8 @@
 
   content = [&dataInStorage] () -> Block {
     EncodingBuffer encoder;
-    size_t valueLength = encoder.prependBytes({dataInStorage[1].getContent().value(),
-                                               dataInStorage[1].getContent().value_size()});
-    valueLength += encoder.prependBytes({dataInStorage[0].getContent().value(),
-                                         dataInStorage[0].getContent().value_size()});
+    size_t valueLength = encoder.prependBytes(dataInStorage[1].getContent().value_bytes());
+    valueLength += encoder.prependBytes(dataInStorage[0].getContent().value_bytes());
     encoder.prependVarNumber(valueLength);
     encoder.prependVarNumber(tlv::Content);
     return encoder.block();