add MultiDict

This commit introduces MultiDict, a multi-dictionary similar to
ODict, but with improved semantics (as in the Headers class).
MultiDict fixes a few issues that were present in the Request/Response
API. In particular, `request.cookies["foo"] = "bar"` has previously been a
no-op, as the cookies property returned a mutable _copy_ of the cookies.

examples/modify_form.py

@@ -1,5 +1,8 @@
 def request(context, flow):
-    form = flow.request.urlencoded_form
-    if form is not None:
-        form["mitmproxy"] = ["rocks"]
-        flow.request.urlencoded_form = form
+    if flow.request.urlencoded_form is not None:
+        flow.request.urlencoded_form["mitmproxy"] = "rocks"
+    else:
+        # This sets the proper content type and overrides the body.
+        flow.request.urlencoded_form = [
+            ("foo", "bar")
+        ]

examples/modify_querystring.py

@@ -1,5 +1,2 @@
 def request(context, flow):
-    q = flow.request.query
-    if q:
-        q["mitmproxy"] = ["rocks"]
-        flow.request.query = q
+    flow.request.query["mitmproxy"] = "rocks"