text: Title: Unintended sharing of `MultipartParseOptions._DEFAULT_HANDLERS` between instances
  Body: I think we should initialize `media_handlers` in `MultipartParseOptions.__init__()` with a copy `MultipartParseOptions._DEFAULT_HANDLERS`, not just assign reference:
  ```python
  >>> from falcon.media.multipart import MultipartParseOptions
  >>> mo1 = MultipartParseOptions()
  >>> mo2 = MultipartParseOptions()
  >>> sorted(mo1.media_handlers)
  ['application/json', 'application/x-www-form-urlencoded']
  >>> mo1.media_handlers.pop('application/x-www-form-urlencoded')
  <falcon.media.urlencoded.URLEncodedFormHandler object at 0x7dce2c618f50>
  >>> mo1.media_handlers
  {'application/json': <falcon.media.json.JSONHandler object at 0x7dce2c7fa990>}
  >>> mo2.media_handlers
  {'application/json': <falcon.media.json.JSONHandler object at 0x7dce2c7fa990>}
  >>> MultipartParseOptions._DEFAULT_HANDLERS
  {'application/json': <falcon.media.json.JSONHandler object at 0x7dce2c7fa990>}
  ```