Fix: encodeDates mutates the object passed to people.set (#479)

[sarmah-rup]

What I hit

I passed a Date to mixpanel.people.set(...) and the Date on my own object silently turned into a string on the next render, which broke a date-picker bound to that object. This is issue #479.

Minimal repro:

var props = { created: new Date('2020-01-02T03:04:05Z') };
mixpanel.people.set(props);
props.created; // "2020-01-02T03:04:05"  <- mutated in place, no longer a Date

Why

_.encodeDates (src/utils.js) writes the encoded strings back onto the object it is given (obj[k] = _.formatDate(v)) and recurses onto the same nested references, so it edits the exact object the caller passed to people.set / set_once (and group set).

Fix

Build a copy instead of mutating the input. The encoded output is identical to before; only the side effect on the caller's object is removed. (_.isObject excludes arrays, so recursion still only descends into plain objects.)

Test

Added cases to tests/unit/utils.js: encoding still works (top-level + nested) and the input object is no longer mutated (top-level + nested). Full tests/unit/* suite passes (684 passing, 0 failing) and eslint ./src is clean.

Closes #479.