fix issue #90

We are static-casting to U, so we really have no reason to use
references.

However, if this comes up again, try applying -ffloat-store to
the target executable, per
    https://github.com/open-source-parsers/jsoncpp/issues/90

src/test_lib_json/jsontest.h

@@ -178,8 +178,8 @@ private:
 
 template <typename T, typename U>
 TestResult& checkEqual(TestResult& result,
-                       const T& expected,
-                       const U& actual,
+                       T expected,
+                       U actual,
                        const char* file,
                        unsigned int line,
                        const char* expr) {