Fixed bug in asDouble but changed its operation. Decimal only.

testing/zstring_test.cpp

@@ -99,5 +99,8 @@ int main(int argc, char **argv) {
    std::cout << "[" << test18.find(".") << "] " << test18.parsed() << ":" << test18.unparsed() << std::endl;
    test18.reset();
    std::cout << "[" << test18.find("x") << "] " << test18.parsed() << ":" << test18.unparsed() << std::endl;
+
+   coreutils::ZString test19("-16473.65476Z");
+   std::cout << test19.asDouble() << ":" << test19.unparsed() << std::endl;
    
 }