Fix ambiguity in stringification

Happening when using clang and templated operators, clang cannot decide
between the operator provided by ReusableStringStream and the one provided
by the value value as both are templates. This is easily solved by calling
the operator<< through the member syntax.

Fixes #1285

include/internal/catch_tostring.h

@@ -105,7 +105,9 @@ namespace Catch {
         typename std::enable_if<::Catch::Detail::IsStreamInsertable<Fake>::value, std::string>::type
             convert(const Fake& value) {
                 ReusableStringStream rss;
-                rss << value;
+                // NB: call using the function-like syntax to avoid ambiguity with
+                // user-defined templated operator<< under clang.
+                rss.operator<<(value);
                 return rss.str();
         }