Fix mount for Node.js >= 15.1.0

I tried the basic example in the README. It failed with a 500
Internal Server Error and the following JSON body:

    {
        "message": "Cannot read properties of undefined (reading 'x-forwarded-proto')",
        "name": "TypeError"
    }

The output from the server process includes a stack trace.
Sanitized a bit, it is:

    TypeError: Cannot read properties of undefined (reading 'x-forwarded-proto')
        at protocol (.../node_modules/paperplane/lib/parseUrl.js:18:14)
        at .../node_modules/ramda/src/converge.js:47:17
        at _map (.../node_modules/ramda/src/internal/_map.js:6:19)
        at .../node_modules/ramda/src/converge.js:46:33
        at f1 (.../node_modules/ramda/src/internal/_curry1.js:18:17)
        at .../node_modules/ramda/src/uncurryN.js:34:21
        at .../node_modules/ramda/src/internal/_curryN.js:37:27
        at .../node_modules/ramda/src/internal/_arity.js:10:19
        at .../node_modules/ramda/src/internal/_pipe.js:3:14
        at .../node_modules/ramda/src/internal/_arity.js:10:19

Yet the docker-based demo app works. That uses Node.js v12.22.12.
When I tried that Node version on the basic example, it also worked.

So, I used `nvm` to do a binary search on versions. I identified
that Node.js v15.1.0 is the first failing version, and every version
I tried that was newer (up to v21.2.0) also failed.

Scouring the Node.js change log revealed that the http module of
v15.1.0 started calculating req.headers lazily. There's a full
discussion in nodejs/node#35281 [1]. Note the referenced issue that
identifies the bug [2].

[1] nodejs/node#35281
[2] nodejs/node#36550

The workaround identified in this comment [3] is not to use the
spread operator or `Object.assign` on the request object. "These
objects are essentially uncloneable."

[3] nodejs/node#36550 (comment)

This is surprisingly tricky to do right. Various Ramda functions
like `merge` (and I think `converge`) do this implicitly, as does
funky's `assocWith`. So to work around this limitation, while
preserving all behavior, I had to resort to (gasp) mutating
procedures instead of pure functions. Not ideal, I know. I'm open to
better ideas.

So, maybe this isn't the best solution, but it least it gets the
example working again on modern Node versions. If it never gets
merged, at least it will be findable via the repo on GitHub.

For reference, to test this, I used a fresh NPM project with only
the paperplane dependency:

    mkdir paperplane
    cd paperplane
    npm init -y
    npm i -S paperplane

In which I added an index.js file with these contents:

    const { compose } = require('ramda')
    const http = require('http')
    const { json, logger, methods, mount, routes } = require('paperplane')

    const cookies = req => ({ cookies: req.cookies || 'none?' })
    const hello   = req => ({ message: `Hello ${req.params.name}!` })

    const app = routes({
      '/'           : methods({ GET: _ => ({ body: 'Hello anon.' }) }),
      '/cookies'    : methods({ GET: compose(json, cookies) }),
      '/hello/:name': methods({ GET: compose(json, hello) })
    })

    http.createServer(mount({ app })).listen(3000, logger)

And finally (with `fd` and `entr` and `httpie` installed), I started
a file-watching auto-test process:

    fd -e js | entr -rcc bash -c "node index.js & http -v get http://localhost:3000/cookies Cookie:foo=bar"

lib/parseCookies.js

@@ -2,4 +2,4 @@ const { assoc } = require('ramda')
 const { parse } = require('cookie')
 
 exports.parseCookies = req =>
-  assoc('cookies', parse(req.headers.cookie || ''), req)
+  assoc('cookies', parse(req && req.headers && req.headers.cookie || ''), req)

lib/parseUrl.js

@@ -15,7 +15,7 @@ const pathParts =
   )
 
 const protocol = req =>
-  req.headers['x-forwarded-proto'] ||
+  req && req.headers && req.headers['x-forwarded-proto'] ||
   req.socket.encrypted ? 'https' : 'http'
 
 exports.parseUrl =