update formDecoder to properly inherit interface

Author: Dean Karn

README.md

@@ -1,6 +1,6 @@
 ##Pure
 <img align="right" src="https://raw.githubusercontent.com/go-playground/pure/master/logo.png">
-![Project status](https://img.shields.io/badge/version-2.2.0-green.svg)
+![Project status](https://img.shields.io/badge/version-2.2.1-green.svg)
 [![Build Status](https://semaphoreci.com/api/v1/joeybloggs/pure/branches/master/badge.svg)](https://semaphoreci.com/joeybloggs/pure)
 [![Coverage Status](https://coveralls.io/repos/github/go-playground/pure/badge.svg?branch=master)](https://coveralls.io/github/go-playground/pure?branch=master)
 [![Go Report Card](https://goreportcard.com/badge/github.com/go-playground/pure)](https://goreportcard.com/report/github.com/go-playground/pure)

form_decoder.go

@@ -13,6 +13,5 @@ type FormDecoder interface {
 
 var (
 	// DefaultDecoder is pure's default form decoder
-	DefaultDecoder             = form.NewDecoder()
-	_              FormDecoder = DefaultDecoder
+	DefaultDecoder FormDecoder = form.NewDecoder()
 )