default cookie path to /

the behavior of set-cookie with no path is surprising:
https://tools.ietf.org/html/rfc6265#section-4.1.2.4

many other frameworks that aren't called bottlepy do this too:
https://github.com/pallets/werkzeug/blob/760a4a95375d1d404b516fd89c2f5dd5c15cf419/werkzeug/wrappers.py#L1034
https://github.com/tornadoweb/tornado/blob/38e493ed4a55e424e5485bd3ffc3c6ccb8a71843/tornado/web.py#L529
https://github.com/cherrypy/cherrypy/blob/2c00069684e3973a79ae6c2ed0433dae8e813602/cherrypy/lib/sessions.py#L856
https://github.com/django/django/blob/544b2ef29f0f2577912f88cf746ae0ca5877b5f8/django/http/response.py#L176

pigwig/request_response.py

@@ -108,7 +108,7 @@ def __init__(self, body=None, code=200, content_type='text/plain', location=None
 			headers.extend(extra_headers)
 		self.headers = headers
 
-	def set_cookie(self, key, value, domain=None, path=None, expires=None, max_age=None, secure=False, http_only=False):
+	def set_cookie(self, key, value, domain=None, path='/', expires=None, max_age=None, secure=False, http_only=False):
 		'''
 		adds a Set-Cookie header
 

pigwig/tests/test_request_response.py

@@ -22,6 +22,23 @@ def test_json(self):
 		self.assertGreater(len(chunks), 1)
 		self.assertEqual(b''.join(chunks), json.dumps(big_obj).encode())
 
+	def test_cookie(self):
+		app = PigWig([])
+		r = Response()
+		r.set_cookie('cow', 'moo')
+		r.set_cookie('duck', 'quack', path='/pond')
+
+		cookies = http.cookies.SimpleCookie()
+		for header, value in r.headers:
+			if header == 'Set-Cookie':
+				cookies.load(value)
+
+		req = Request(app, None, None, None, None, None, cookies, None)
+		self.assertEqual(req.cookies['cow'].value, 'moo')
+		self.assertEqual(req.cookies['cow']['path'], '/')
+		self.assertEqual(req.cookies['duck'].value, 'quack')
+		self.assertEqual(req.cookies['duck']['path'], '/pond')
+
 	def test_secure_cookie(self):
 		app = PigWig([], cookie_secret=b'a|b')
 		req = Request(app, None, None, None, None, None, None, None)