Add LocalForm to make testing with form data more ergonomic

[ELD]

This adds the testing improvement described in #1591.

The transformation of the form fields into the resulting body data is a bit clumsy and could probably use some improvement/cleanup. I believe the multipart form translation is correct, but there's only one, narrow test case that confirms it works as expected.

Additionally, I was unable to get something like this to work without ugly ergonomics:

let client = Client::tracked(rocket).unwrap();
let response = client.post("/")
    .form(&[("field", "value"), ("is it", "a cat?")])
    .dispatch();

I might be missing something, but attempting to do something like this:

impl<'v, F: Into<ValueField<'v>>, I: Iterator<Item = F>> From<I> for LocalForm { /* .. */ }

requires the caller to call the method in a way similar to this:

client.post("/")
    .form(&[("field", "value"), ("is it", "a cat?")].iter().cloned());

due to a slice reference not being coerced into an iterator and when it is iterated over, it returns &(&str, &str). ValueField doesn't have a present From<..> impl for &(&str, &str), so this is not possible.

Also, docs are likely not complete/elaborate enough at this stage, but an attempt at documenting relevant methods/structs was made in this first cut.

Please pick it apart so I can make the implementation better! :)

[SergioBenitez]

The code is handling percent-encoding incorrectly. It's decoding when it shouldn't be and failing to encode when it should be. Here are the conditions:

• If a provided string is percent-encoded, it should be left alone.
• If a provided string is not percent-encoded, it should be percent-encoded.

Specifically, note that for some s, encode(decode(s)) != s (for example, %).

Note that we have the UriDisplay<Query> trait that already knows how to do this for a bunch of types. We should probably use it here. Perhaps better type signatures are:

pub fn field<N, V>(mut self, name: N, value: V) -> Self
    where N: Into<NameBuf<'v>>, V: UriDisplay<Query>,

Let's start by fixing the incorrect code and style and we'll go from there.

[ELD]

@SergioBenitez This should be ready for another pass.

I updated the style to better match Rocket's. I think it's mostly there, but there might be some minor style issues remaining.

I changed the writing of the body data to a single Vec<u8> with the write! macro. There's a couple places where I call format!. There's probably a better way than what I did, so any help there would be great!

Error handling is a little weird as it currently silently outputs an empty buffer if the writing failed for some reason. This probably isn't a good user experience because an error would result in empty body data for the form request. Any pointers on how to do better error handling would be great!

[slyons]

Has anyone had a chance to look at this yet?