routing: Transparently convert a MultiDict containing parameters.

[houseofsuns]

Sorry to nag once more w.r.t. the multiple values for URL building, but I think I found a way to make everybody happy. The following should allow passing in MultiDicts and receiving the desired behaviour without getting any regressions.
This should be ready to merge (as in there is test and documentation).

This allows to pass a MultiDict and get the intuitively expected result
(that is multiple parameters, where multiple values are present). This
allows to recreate a URL as follows:

• create a URL with multiple values for a parameter
• use Map.match() to extract them into a MultiDict
• recreate the URL with Map.build() handing in the MultiDict

The last step would fail before this commit (in that it would reproduce only
one of the values for each parameter). Also this avoids the collateral
damage introduced by the last attempt at allowing MultiDicts.

This zaps one of the test cases in test_basic_building to reflect the
changed behaviour.

[houseofsuns]

Hm, I messed up the layout above ...

[untitaker]

You can pass in multiple query params by providing a list of values as value. Isn't that good enough?

[houseofsuns]

I have no strong feelings here, but I found the point that build() is then inverse to match() a big plus. For me this is more an enhancement/sugar coating so that everything works as I would intuitively expect it.

Also if not this change, I will have to modify some of my code to use lists of values, so I thought I might try here first, for the benefit of the general public. :)

[houseofsuns]

I just saw the failure and fixed the syntax to be compatible to 2.6

[houseofsuns]

Tentative bump. What about this, it looks like I'll actually not need this anymore, but I think it would be a sensible change anyway. :)

[davidism]

Issue history so I can keep this straight:

• url building with values supplied by MultiDict does not give multiple values #658 was the initial request to support multiple values when building.
• routing.build: Allow multiple values for the same field #660 converted the values to a MultiDict unconditionally.
• Fix regression from #660 #667 fixed an issue routing.build: Allow multiple values for the same field #660 caused when passing dicts.
• url_for with a tuple does not work! #709 reported an issue with being able to indicate a single, iterable value vs multiple values.
• Fix regression in routing system #710 reverted routing.build: Allow multiple values for the same field #660 to return to the previous behavior.

[davidism]

Continued in #1310