Add ToFormData class

[fizruk]

Form data is halfway between ParamSchema and Schema with an extra property that it can handle SwaggerFile. I think it makes sense to introduce a class ToFormData for that. Instances of this class can be derived generically for records where each field has a ParamSchema instance.

The correspondence of type-level and value-level classes is this:

• ToParamSchema — ToHttpApiData;
• ToFormData — ToFormUrlEncoded;
• ToSchema — ToJSON.

Having ToFormData class with Generic-based deriving should reduce a significant amount of boilerplate.

The problem I currently have with ToFormData class is that swagger2 model does not allow for this class to be pretty:

type FormDataParam =
  ( ParamSchema ParamOtherSchema
  , Maybe Bool -- required?
  , Maybe Bool -- allow empty values?
  )

class ToFormData a where
  toFormData :: proxy a -> HashMap ParamName FormDataParam

Basically, FormDataParam should be just Param with the restriction that ParamLocation is ParamFormData. But we cannot specify that constraint in current model.
Actually I don't know if we can specify it at all (without resorting to existentials like SomeParam).

For now we can define a data type for form data parameters together with conversion function:

data FormDataParam = FormDataParam
  { formDataParamDescription     :: Maybe Text
  , formDataParamRequired        :: Maybe Bool
  , formDataParamAllowEmptyValue :: Maybe Bool
  , formDataParamParamSchema     :: ParamSchema ParamOtherSchema
  }

formDataParamToParam :: FormDataParam -> Param
formDataParamToParam = ...

@dmjio what do you think about this hacky solution?

[fizruk]

Forms are now in http-api-data-0.3!

[fizruk]

Closing this as I don't currently see much use for this class.