Zeronet UI does not work without HTTP Accept header

### Step 1: Please describe your environment

* ZeroNet version: 0.6.2
* Operating system: Linux
* Web browser: Firefox
* Tor status: enabled
* Opened port: yes
* Special configuration: I2P Tunnel

### Step 2: Describe the problem:

Zeronet Ui expects HTTP `Accept` header to contain at least `text/html`. I2P tunnels remove `Accept` header from the request, which does not allow to access Zeronet due to the following check:
https://github.com/HelloZeroNet/ZeroNet/blob/master/src/Ui/UiRequest.py#L283

#### Steps to reproduce:

1. Send HTTP request without `Accept` HTTP header to Zeronet UI address

#### Observed Results:

Zeronet returns HTTP 403 `Invalid Accept header to load wrapper` error.

#### Expected Results:

Zeronet ignores absence of HTTP `Accept` header and continue to work.