make ToString() get local date

**Describe the bug**
I am sending the data to the server directly but it is taking by default UTC time so it generates a bug when making a database filter with the date time, maybe is it because when parsing datetime it make it by default UTC ?

`
toString() {
return this.$d.toUTCString()
}
`

**Expected behavior**
Consider to put by default local datetime

**Information**
- Day.js Version [e.g. v1.11.13]
- OS: [e.g. iOS]
- Browser [e.g. chrome 62]
- Time zone: [e.g. GMT-07:00 DST (Pacific Daylight Time)]

Review the toString() entry point shown in the issue and the library's existing date and timezone behavior. Confirm how local and UTC values are represented, then verify that the default string output uses local datetime without breaking documented serialization expectations.

Written by the indexing model from the issue text.