Fix content-length calcuation in Rack:Response#write

When `Rack::Response` is initialized with an Array, it incorrectly
increments its internal `@length` value and emitted content-length
header on every subsequent write. The more times `write` is called, the
more the error accumulates.

This commit fixes the accumulation bug, and fixes/adds specs to properly
test the scenario where `write` is used multiple times.

lib/rack/response.rb

@@ -321,6 +321,8 @@ def buffered_body!
             @body.each do |part|
               @length += part.to_s.bytesize
             end
+
+            @buffered = true
           elsif @body.respond_to?(:each)
             # Turn the user supplied body into a buffered array:
             body = @body

test/spec_response.rb

@@ -408,7 +408,7 @@ def object_with_each.each
     status.must_equal 404
   end
 
-  it "correctly updates content-type when writing when not initialized with body" do
+  it "correctly updates content-length when writing when initialized without body" do
     r = Rack::Response.new
     r.write('foo')
     r.write('bar')
@@ -419,20 +419,24 @@ def object_with_each.each
     header['content-length'].must_equal '9'
   end
 
-  it "correctly updates content-type when writing when initialized with body" do
-    obj = Object.new
-    def obj.each
-      yield 'foo'
-      yield 'bar'
-    end
-    ["foobar", ["foo", "bar"], obj].each do
-      r = Rack::Response.new(["foo", "bar"])
-      r.write('baz')
-      _, header, body = r.finish
-      str = "".dup; body.each { |part| str << part }
-      str.must_equal "foobarbaz"
-      header['content-length'].must_equal '9'
-    end
+  it "correctly updates content-length when writing when initialized with Array body" do
+    r = Rack::Response.new(["foo"])
+    r.write('bar')
+    r.write('baz')
+    _, header, body = r.finish
+    str = "".dup; body.each { |part| str << part }
+    str.must_equal "foobarbaz"
+    header['content-length'].must_equal '9'
+  end
+
+  it "correctly updates content-length when writing when initialized with String body" do
+    r = Rack::Response.new("foo")
+    r.write('bar')
+    r.write('baz')
+    _, header, body = r.finish
+    str = "".dup; body.each { |part| str << part }
+    str.must_equal "foobarbaz"
+    header['content-length'].must_equal '9'
   end
 
   it "doesn't return invalid responses" do