Fix reader bug: replace asCString() with asString()

[dota17]

When I add the testcase for Reader, I find a bug in

jsoncpp/src/lib_json/json_reader.cpp

Line 470 in d6c4a8f

name = String(numberName.asCString());

The testcase is as follows:

JSONTEST_FIXTURE_LOCAL(ReaderTest, allowNumericKeysTest) {
  Json::Features features;
  features.allowNumericKeys_ = true;
  setFeatures(features);
  checkParse(R"({ 123 : "abc" })");
}

The result is as follows:

Testing MemberTemplateAs/BehavesSameAsNamedAs: OK
Testing MemberTemplateIs/BehavesSameAsNamedIs: OK
* Detail of ReaderTest/allowNumericKeys test failure:
../src/test_lib_json/jsontest.cpp(244): Unexpected exception caught:
  in Json::Value::asCString(): requires stringValue
113/114 tests passed (1 failure(s))
-------

By analyzing related code, I find the asCString() is not a suitable interface for NumericKeys.

const char* Value::asCString() const {
  JSON_ASSERT_MESSAGE(type() == stringValue,
                      "in Json::Value::asCString(): requires stringValue");
  if (value_.string_ == nullptr)
    return nullptr;
  unsigned this_len;
  char const* this_str;
  decodePrefixedString(this->isAllocated(), this->value_.string_, &this_len,
                       &this_str);
  return this_str;
}

numberName value is a number value(type is realvalue/uintvalue) and asCString() can not convert the numbervalue to string. We should replace asCString() with asString().

Modift content is shown in #1122