Fix gateway writable state check

[fooock]

See https://github.com/ipfs/public-gateway-checker/issues/3#issuecomment-477844332

[fooock]

Seems that Infura has weird behavior when I make a post request to check gateway writable state. If I execute curl -v -X POST https://ipfs.infura.io/ipfs/:

> POST /ipfs/ HTTP/1.1
> Host: ipfs.infura.io
> User-Agent: curl/7.60.0
> Accept: */*
>
< HTTP/1.1 405 Method Not Allowed
< Date: Sat, 30 Mar 2019 18:09:44 GMT
< Content-Type: text/plain; charset=utf-8
< Content-Length: 41
< Connection: keep-alive
< Server: nginx
<
* Connection #0 to host ipfs.infura.io left intact
Method POST not allowed: read only access

But Infura has writable state because I can add files througth the :5001/api/v0/add api service.

[fooock]

The question in Infura community docs: https://community.infura.io/t/post-request-to-ipfs-gateway-returns-405/454