Allow HeaderHash#fetch to respect case insensitivity

It just makes good sense in general, but it specifically makes the
proximate fix for rack#1629 a lot cleaner.

lib/rack/utils.rb

@@ -474,6 +474,14 @@ def include?(k)
       alias_method :member?, :include?
       alias_method :key?, :include?
 
+      def fetch(*args)
+        begin
+          super(args.first)
+        rescue KeyError
+          super(@names[args.first.downcase], *args[1..-1])
+        end
+      end
+
       def merge!(other)
         other.each { |k, v| self[k] = v }
         self

test/spec_utils.rb

@@ -639,6 +639,24 @@ def initialize(*)
     h.wont_include 'ETag'
   end
 
+  it "fetches values via case-insensitive keys" do
+    h = Rack::Utils::HeaderHash.new("Content-MD5" => "d5ff4e2a0 ...")
+    v = h.fetch("content-MD5", "nope")
+    v.must_equal "d5ff4e2a0 ..."
+  end
+
+  it "fetches values via case-insensitive keys without defaults" do
+    h = Rack::Utils::HeaderHash.new("Content-MD5" => "d5ff4e2a0 ...")
+    v = h.fetch("content-MD5")
+    v.must_equal "d5ff4e2a0 ..."
+  end
+
+  it "correctly raises an exception on fetch for a non-existent key" do
+    h = Rack::Utils::HeaderHash.new("Content-MD5" => "d5ff4e2a0 ...")
+
+	 -> { h.fetch("Set-Cookie") }.must_raise(KeyError)
+  end
+
   it "create deep HeaderHash copy on dup" do
     h1 = Rack::Utils::HeaderHash.new("Content-MD5" => "d5ff4e2a0 ...")
     h2 = h1.dup