interest+data: add string_view overloads for convenience

Also, explicitly declare the nullptr_t overloads as deleted, so that
users get a clear compilation error instead of a potentially confusing
"ambiguous overload" if they try to pass nullptr to these functions.

Change-Id: I399efcbd698a0d3f7e29219d21eb1a53a1d71070
diff --git a/tests/unit/security/certificate.t.cpp b/tests/unit/security/certificate.t.cpp
index facb261..a44ddfb 100644
--- a/tests/unit/security/certificate.t.cpp
+++ b/tests/unit/security/certificate.t.cpp
@@ -315,8 +315,8 @@
 R"TXT(Certificate Name:
   /ndn/test/identity/KEY/%C7G%3A%D6%12P%B5%F0/self/v=1650251820652
 Public Key:
-  Key Type: Unknown (23 bytes)
-  bm90IGEgdmFsaWQgcHVibGljIGtleQA=
+  Key Type: Unknown (22 bytes)
+  bm90IGEgdmFsaWQgcHVibGljIGtleQ==
 Validity:
   Not Before: 1970-01-01T00:00:00
   Not After: 2042-04-13T03:17:00
@@ -326,9 +326,8 @@
   Self-Signed: yes
 )TXT");
 
-  const uint8_t notAKey[] = "not a valid public key";
   Certificate cert4(cert3);
-  cert4.setContent(notAKey);
+  cert4.setContent("not a valid public key"sv);
   BOOST_CHECK_EQUAL(boost::lexical_cast<std::string>(cert4), expected4);
 
   const std::string expected5(
diff --git a/tests/unit/security/key-chain.t.cpp b/tests/unit/security/key-chain.t.cpp
index d0b717f..c154851 100644
--- a/tests/unit/security/key-chain.t.cpp
+++ b/tests/unit/security/key-chain.t.cpp
@@ -711,7 +711,7 @@
   BOOST_CHECK_THROW(signerKeyChain.makeCertificate(request, signerParams), std::invalid_argument);
 
   // empty content
-  request.setContent(span<uint8_t>{});
+  request.setContent("");
   BOOST_CHECK_THROW(signerKeyChain.makeCertificate(request, signerParams), std::invalid_argument);
 }