Dividing a number may return a float, which will cause a `TypeError` once we add type hints.

The value was `1` originally, when timeouts were introduced along with `SimplePie_File` in
simplepie@5bf1814

We could consider hardcoding it again but we still want to keep it proportional to the master timeout
to allow people with slow connection to increase it, as introduced in
simplepie@fe7e333

In fact, reducing the timeout probably does not make sense in the first place.
Even if we assume majority of cached resources will not get outdated,
waiting for server to reply `HTTP 304 Not Modified` can still take time
(domain name resolution can be particularly slow).
And if it turns out that the cached version is outdated,
the web server will try to send an updated version.
Having too low timeout can mean the request will be terminated,
only to be started again with the original timeout.

The timeout reduction was presumably done to prevent
fetching a cached resource from an unresponsive server without `force_cache_fallback`
from essentially doubling the timeout due to re-running the failed request.

Let’s use the original timeout and prevent repeat requests instead.

- Drop unused variable
- Add white space for clarity

PSR-7 `MessageInterface` expects values of each header is always a list.
But concatenating multiple instances of the same header like we do now
is a lossy operation so we cannot confidently split them.

To allow us to use PSR-7-like semantics internally,
let’s add PSR-7 mode to the HTTP parser that will avoid the imploding.

`Client` is sort of like `ClientInterface` from PSR-18 but the `request()` method expects request parameters as discrete arguments rather than wrapped in PSR-7 `RequestInterface` like `ClientInterface::sendRequest()` does.

`Response` is a pared-down `ResponseInterface` from PSR-7. There are also other notable differences from PSR-7:
- Method names use snake_case.
- The class also provides the URI of the final HTTP request after redirects through `get_final_requested_uri()` and permanent URI of the requested resource through `get_permanent_uri()`
- Body is returned as a string rather than stream in `get_body_content()`.
- `get_headers()` does not have to preserve the exact case in which headers were originally specified.

https://www.php-fig.org/psr/psr-7/
https://www.php-fig.org/psr/psr-18/

Acked-by: Jan Tojnar <jtojnar@gmail.com>

Implementing the PSR-7 inspired `Response` interface gets us one step
closer to pluggable HTTP implementation.

Most of the complexity comes from the fact that PSR-7 `MessageInterface`
expects values of each header to always be a list, while `File` stored
each as a single strings. But we cannot change the type of the `$headers`
property without breaking backwards compatibility (especially for `File`
subclasses that modify the headers), and we cannot just confidently split
the header lines in the methods, since the concatenating multiple instances
of the same header like `HTTP\Parser` currently does is a lossy operation.

As a result, we have to check if `$headers` property changed every time
we access the internal header state.

Co-authored-by: Jan Tojnar <jtojnar@gmail.com>

Mostly extract repeat use of values from `File` object into local variables.
This will make the changes in the next commit cleaner.

Acked-by: Jan Tojnar <jtojnar@gmail.com>

Introduces a trivial `Client` that just creates a `File` object.

Acked-by: Jan Tojnar <jtojnar@gmail.com>

Acked-by: Jan Tojnar <jtojnar@gmail.com>

Acked-by: Jan Tojnar <jtojnar@gmail.com>

Acked-by: Jan Tojnar <jtojnar@gmail.com>

Methods should be used instead.

Acked-by: Jan Tojnar <jtojnar@gmail.com>