Fix crash with char[] default values

Closes #249

Reported-by: @pfeatherstone
Signed-off-by: Sean Robinson <sean.robinson@scottsdalecc.edu>

include/argparse/argparse.hpp

@@ -408,6 +408,10 @@ public:
     return *this;
   }
 
+  Argument &default_value(const char *value) {
+    return default_value(std::string(value));
+  }
+
   Argument &required() {
     m_is_required = true;
     return *this;

test/CMakeLists.txt

@@ -33,6 +33,7 @@ file(GLOB ARGPARSE_TEST_SOURCES
     test_container_arguments.cpp
     test_const_correct.cpp
     test_default_args.cpp
+    test_default_value.cpp
     test_get.cpp
     test_help.cpp
     test_invalid_arguments.cpp

test/test_default_value.cpp

@@ -0,0 +1,21 @@
+#include <argparse/argparse.hpp>
+#include <doctest.hpp>
+#include <string>
+
+using doctest::test_suite;
+
+TEST_CASE("Use a 'string' default value" * test_suite("default_value")) {
+  argparse::ArgumentParser program("test");
+
+  SUBCASE("Use a const char[] default value") {
+    program.add_argument("--arg").default_value("array of char");
+    REQUIRE_NOTHROW(program.parse_args({"test"}));
+    REQUIRE(program.get("--arg") == std::string("array of char"));
+  }
+
+  SUBCASE("Use a std::string default value") {
+    program.add_argument("--arg").default_value(std::string("string object"));
+    REQUIRE_NOTHROW(program.parse_args({"test"}));
+    REQUIRE(program.get("--arg") == std::string("string object"));
+  }
+}