fix(StringContent): compareString problem

strcmp returns wrong result

src/eckit/value/StringContent.cc

@@ -66,7 +66,7 @@ int StringContent::compare(const Content& other) const {
 }
 
 int StringContent::compareString(const StringContent& other) const {
-    return ::strcmp(value_.c_str(), other.value_.c_str());
+    return value_.compare(other.value_);
 }
 
 void StringContent::value(std::string& s) const {