Make query string parsing conform to URL spec

The URL spec section 5.1.3.3 specifies that if = is not present
in the byte sequence, it should be treated as if the byte sequence
is the name of the tuple and the value is the empty string.

This affects all parameters without =, not just arrays:

```ruby
Rack::Utils.parse_nested_query("foo[bar]&baz[]&quux")
{"foo"=>{"bar"=>nil}, "baz"=>[nil], "quux"=>nil} # Before
{"foo"=>{"bar"=>""}, "baz"=>[""], "quux"=>""}    # After
```

Fixes rack#1696

CHANGELOG.md

@@ -16,6 +16,7 @@ All notable changes to this project will be documented in this file. For info on
 ### Changed
 
 - BREAKING CHANGE: Require `status` to be an Integer. ([#1662](https://github.com/rack/rack/pull/1662), [@olleolleolle](https://github.com/olleolleolle))
+- BREAKING CHANGE: Query parsing now treats parameters without `=` as having the empty string value instead of nil value, to conform to the URL spec. ([#1696](https://github.com/rack/rack/issues/1696), [@jeremyevans](https://github.com/jeremyevans))
 - Relax validations around `Rack::Request#host` and `Rack::Request#hostname`. ([#1606](https://github.com/rack/rack/issues/1606), [@pvande](https://github.com/pvande))
 - Removed antiquated handlers: FCGI, LSWS, SCGI, Thin. ([#1658](https://github.com/rack/rack/pull/1658), [@ioquatix](https://github.com/ioquatix))
 - Removed options from `Rack::Builder.parse_file` and `Rack::Builder.load_file`. ([#1663](https://github.com/rack/rack/pull/1663), [@ioquatix](https://github.com/ioquatix))

lib/rack/query_parser.rb

@@ -83,6 +83,7 @@ def normalize_params(params, name, v, depth)
       name =~ %r(\A[\[\]]*([^\[\]]+)\]*)
       k = $1 || ''
       after = $' || ''
+      v ||= String.new
 
       if k.empty?
         if !v.nil? && name == "[]"

test/spec_utils.rb

@@ -135,7 +135,7 @@ def assert_nested_query(exp, act)
 
   it "parse nested query strings correctly" do
     Rack::Utils.parse_nested_query("foo").
-      must_equal "foo" => nil
+      must_equal "foo" => ""
     Rack::Utils.parse_nested_query("foo=").
       must_equal "foo" => ""
     Rack::Utils.parse_nested_query("foo=bar").
@@ -152,7 +152,7 @@ def assert_nested_query(exp, act)
     Rack::Utils.parse_nested_query("&foo=1&&bar=2").
       must_equal "foo" => "1", "bar" => "2"
     Rack::Utils.parse_nested_query("foo&bar=").
-      must_equal "foo" => nil, "bar" => ""
+      must_equal "foo" => "", "bar" => ""
     Rack::Utils.parse_nested_query("foo=bar&baz=").
       must_equal "foo" => "bar", "baz" => ""
     Rack::Utils.parse_nested_query("my+weird+field=q1%212%22%27w%245%267%2Fz8%29%3F").
@@ -162,19 +162,19 @@ def assert_nested_query(exp, act)
       must_equal "pid=1234" => "1023", "a" => "b"
 
     Rack::Utils.parse_nested_query("foo[]").
-      must_equal "foo" => [nil]
+      must_equal "foo" => [""]
     Rack::Utils.parse_nested_query("foo[]=").
       must_equal "foo" => [""]
     Rack::Utils.parse_nested_query("foo[]=bar").
       must_equal "foo" => ["bar"]
     Rack::Utils.parse_nested_query("foo[]=bar&foo").
-      must_equal "foo" => nil
+      must_equal "foo" => ""
     Rack::Utils.parse_nested_query("foo[]=bar&foo[").
-      must_equal "foo" => ["bar"], "foo[" => nil
+      must_equal "foo" => ["bar"], "foo[" => ""
     Rack::Utils.parse_nested_query("foo[]=bar&foo[=baz").
       must_equal "foo" => ["bar"], "foo[" => "baz"
     Rack::Utils.parse_nested_query("foo[]=bar&foo[]").
-      must_equal "foo" => ["bar", nil]
+      must_equal "foo" => ["bar", ""]
     Rack::Utils.parse_nested_query("foo[]=bar&foo[]=").
       must_equal "foo" => ["bar", ""]
 
@@ -185,6 +185,8 @@ def assert_nested_query(exp, act)
     Rack::Utils.parse_nested_query("foo[]=bar&baz[]=1&baz[]=2&baz[]=3").
       must_equal "foo" => ["bar"], "baz" => ["1", "2", "3"]
 
+    Rack::Utils.parse_nested_query("x[y][z]").
+      must_equal "x" => { "y" => { "z" => "" } }
     Rack::Utils.parse_nested_query("x[y][z]=1").
       must_equal "x" => { "y" => { "z" => "1" } }
     Rack::Utils.parse_nested_query("x[y][z][]=1").
@@ -338,7 +340,7 @@ def initialize(*)
   end
 
   it 'performs the inverse function of #parse_nested_query' do
-    [{ "foo" => nil, "bar" => "" },
+    [{ "bar" => "" },
       { "foo" => "bar", "baz" => "" },
       { "foo" => ["1", "2"] },
       { "foo" => "bar", "baz" => ["1", "2", "3"] },