encoding: make Block convertible to span

Change-Id: Idb501467d1c56b95e7fdb76acda63b919fe98f85
diff --git a/tests/unit/security/detail/certificate-bundle-decoder.t.cpp b/tests/unit/security/detail/certificate-bundle-decoder.t.cpp
index 630aaec..1199994 100644
--- a/tests/unit/security/detail/certificate-bundle-decoder.t.cpp
+++ b/tests/unit/security/detail/certificate-bundle-decoder.t.cpp
@@ -99,7 +99,7 @@
 {
   // First segment contains first 250 bytes of cert1
   Data d;
-  d.setContent(make_span(certBlock1.wire(), 250));
+  d.setContent(make_span(certBlock1).first(250));
 
   // Second segment contains the rest of cert1 and the first 100 bytes of cert2
   auto buf = std::make_shared<Buffer>(certBlock1.begin() + 250, certBlock1.end());