Fix url encoded requests

[donny-dont]

During the migration we lost support for application/x-www-form-urlencoded. Wanted to know if there is any preference on how to fix this @nex3 .

I'm thinking it might make sense to have another constructor for Request.

[nex3]

Can we support people just passing a Map<String, String> to the existing constructors?

[donny-dont]

I'm just not sure how were going to get the content-type correct. Still digging through 0.11.x to see how it might be done.

[nex3]

We should be able to manipulate headers in the constructor body—we do similar stuff with the encoding already.

[donny-dont]

Adding some tests with #102 so this can be implemented properly.

[donny-dont]

@nex3 are we going to need to add a read method to get back the map?

[nex3]

Let's hold off on that for now until we have a more coherent answer for how to represent a map with multiple values for a given key.

[donny-dont]

@nex3 @kevmoo how would you guys feel if Body contained the content type? By the time _adjustHeaders is hit in message.dart its already a Body so there isn't a way to determine it without modifying that bit.

[nex3]

I think we can handle it more cleanly than changing the shape of the type. What if we just add a bool formFields argument to new Message._() and adjustHeaders() to track whether the body was originally a map?

[donny-dont]

@nex3 you can close this