Allow a referened context document to not contain @context on expansion.

lib/json/ld/evaluation_context.rb

@@ -118,8 +118,7 @@ def parse(context)
         # Load context document, if it is a string
         begin
           ctx = JSON.load(context)
-          raise JSON::LD::InvalidContext::Syntax, "missing @context" unless ctx.is_a?(Hash) && ctx["@context"]
-          parse(ctx["@context"])
+          parse(ctx["@context"] || {})
         rescue JSON::ParserError => e
           debug("parse") {"Failed to parse @context from remote document at #{context}: #{e.message}"}
           raise JSON::LD::InvalidContext::Syntax, "Failed to parse remote context at #{context}: #{e.message}" if @options[:validate]

spec/evaluation_context_spec.rb

@@ -43,6 +43,11 @@ def @ctx.content_type; "application/ld+json"; end
           "avatar"   => "http://xmlns.com/foaf/0.1/avatar"
         }, @debug)
       end
+
+      it "allows a non-existing @context" do
+        ec = subject.parse(StringIO.new("{}"))
+        ec.mappings.should produce({}, @debug)
+      end
     end
 
     context "EvaluationContext" do