BsDatepicker min-max dates not react on resetting

[timaxapa]

Bug description or feature request:

Once defined minDate or maxDate for bsDatepicker there is no way to reset them

Plunker/StackBlitz that reproduces the issue:

https://stackblitz.com/edit/angular-r6jvsg

Versions of ngx-bootstrap, Angular, and Bootstrap:

ngx-bootstrap: 2.0.0-beta.8
Angular: 4 or 5
Bootstrap: 3.3

Build system: Angular CLI, System.js, webpack, starter seed:

Angular CLI: 1.4

Workaround:
Custom wrapper with code

    if (changes.minDate && !changes.minDate.isFirstChange() && !this.minDate) {
      this.minDate = new Date(0);
    }
    if (changes.maxDate && !changes.maxDate.isFirstChange() && !this.maxDate) {
      this.maxDate = new Date(8640000000000000);
    }

I am sexy and I know it

[valorkin]

indeed

    this._config = Object.assign({}, this._config, this.bsConfig, {
      value: this._bsValue,
      isDisabled: this.isDisabled,
      minDate: this.minDate || this._config.minDate,
      maxDate: this.maxDate || this._config.maxDate
    });

this overwrites config values and have a fallback to them

[valorkin]

it should be enough to remove || this._config.minDate, and maxDate and will work as expected
nice reason for small PR
this code repeats in date and range picker :D