Allow override Urlencoded encoder implementation

``` ruby
HTTP::FormData::Urlencoded.encoder = lambda do |enum|
  unescaped_chars = /[^a-z0-9\-\.\_\~]/i
  parts = []

  enum.each do |k, v|
    k = ::URI::DEFAULT_PARSER.escape(k.to_s, unescaped_chars)

    if v.nil?
      parts << k
    elsif v.respond_to?(:to_ary)
      v.to_ary.each do |vv|
        vv = ::URI::DEFAULT_PARSER.escape(v.to_s, unescaped_chars)
        parts << "#{k}=#{vv}"
      end
    else
      v = ::URI::DEFAULT_PARSER.escape(v.to_s, unescaped_chars)
      parts << "#{k}=#{v}"
    end
  end

  parts.join("&")
end
```

lib/http/form_data/urlencoded.rb

@@ -11,10 +11,17 @@ module FormData
     class Urlencoded
       include Readable
 
+      class << self
+        attr_writer :encoder
+
+        def encoder
+          @encoder ||= ::URI.method(:encode_www_form)
+        end
+      end
+
       # @param [#to_h, Hash] data form data key-value Hash
       def initialize(data)
-        uri_encoded_data = ::URI.encode_www_form FormData.ensure_hash(data)
-        @io = StringIO.new(uri_encoded_data)
+        @io = StringIO.new(self.class.encoder.call(FormData.ensure_hash(data)))
       end
 
       # Returns MIME type to be used for HTTP request `Content-Type` header.

spec/lib/http/form_data/urlencoded_spec.rb

@@ -55,4 +55,13 @@
       expect(form_data.read).to eq form_data.to_s
     end
   end
+
+  describe ".encoder=" do
+    before { described_class.encoder = ::JSON.method(:dump) }
+    after  { described_class.encoder = ::URI.method(:encode_www_form) }
+
+    it "switches form encoder implementation" do
+      expect(form_data.to_s).to eq('{"foo[bar]":"test"}')
+    end
+  end
 end