add special case for application/x-www-form-encoded bodies

[urandom2]

Form encoding is a well structured, flat key-value pair interface. Much like application/json, we should be able to break out a case for a more ergonomic interface. I propose the following:

---

1. If you have more than one request body type, meaning more than one media
   type, you will have a generic handler of this form:

    AddPet(ctx context.Context, contentType string, body io.Reader)
   

2. If you have only a JSON request body, you will get:

    AddPet(ctx context.Context, body NewPet)
   

3. If you have only a form url encoding request body, you will get:

    AddPet(ctx context.Context, body NewPet)
   

4. If you have multiple request body types, which include a JSON or form encoded type you will get multiple functions. We've chosen to give the JSON version a shorter name, as we work with JSON and don't want to wear out our keyboards.

    AddPet(ctx context.Context, body NewPet)
    AddPetForm(ctx context.Context, body NewPet)
    AddPetWithBody(ctx context.Context, contentType string, body io.Reader)
   

[deepmap-marcinr]

I'm thinking the fix for this kind of issue is to allow the registering of unmarshal helper functions keyed off content-type, and we simply provide some pre-defined ones, but users are free to add their own.

That would be a very breaking API change, though. I'm going to start filing tasks for a V2 version of this library, where we will also get strict about semver for generated code.