Guzzle plaintext response error

[dumityty]

This is a PR to fix the issue raised in #152 which I've also encountered after applying the patch from #139 to be able to use the WorkersKV endpoints.

The fix is quite basic and there might be a better of doing this, but from what I can see whenever there's either success or error responses they are always in JSON format, except for the one random endpoint to 'Read key-value pair ' (https://api.cloudflare.com/#workers-kv-namespace-read-key-value-pair) which returns the value of the key in plain text.
So to get around that I just checked if the first character of the response is a { or not, and if it's not then skip all the json code below otherwise it fails with a 'json syntax' error since it's a plain text.

I ran all the tests and they passed, however I didn't write a new one as the change is to the Guzzle adapter which already has all the tests.
I guess I could try writing one to cover the 'plain text' response from the API.

[deuill]

I'll need to investigate this, as json_decode on the responses from that sample endpoint (e.g. "Some Value", note the double-quotes) are correctly decoded from a bare JSON string into a PHP string. That is to say, the responses for that endpoint are indeed valid JSON, albeit not wrapped in an object as is typically the case.

[deuill]

I've opened a perhaps more concrete fix to the issue you're seeing here under #164.