Require quotes around string values in set

This change makes it more consistent with string key values: these also
have to be quoted. It will also help me reuse the leaf_data parser for
the key values.

Change-Id: I5647403648830614d308073be6fb4f1f5469dcb2
diff --git a/src/utils.cpp b/src/utils.cpp
index cd0858f..d0e8639 100644
--- a/src/utils.cpp
+++ b/src/utils.cpp
@@ -67,6 +67,8 @@
         return "an identity";
     case yang::LeafDataTypes::LeafRef:
         return "a leafref";
+    case yang::LeafDataTypes::Binary:
+        return "a base64-encoded binary value";
     default:
         throw std::runtime_error("leafDataTypeToString: unsupported leaf data type");
     }