Satisfy IO#readpartial API

IO#readpartial allows for a second "outbuf" param which some streaming usages
expect, so support it here to allow using response bodies anywhere IO can be.

lib/http/connection.rb

@@ -83,17 +83,19 @@ def send_request(req)
     #
     # @return [String] data chunk
     # @return [nil] when no more data left
-    def readpartial(size = BUFFER_SIZE)
+    def readpartial(size = BUFFER_SIZE, outbuf = nil)
       return unless @pending_response
 
       chunk = @parser.read(size)
-      return chunk if chunk
 
-      finished = (read_more(size) == :eof) || @parser.finished?
-      chunk    = @parser.read(size)
-      finish_response if finished
+      unless chunk
+        finished = (read_more(size) == :eof) || @parser.finished?
+        chunk    = @parser.read(size)
+        finish_response if finished
+      end
 
-      chunk || "".b
+      chunk ||= "".b
+      outbuf ? outbuf.replace(chunk) : chunk
     end
 
     # Reads data from socket up until headers are loaded

spec/lib/http/connection_spec.rb

@@ -62,5 +62,13 @@
       end
       expect(buffer).to eq "1234567890"
     end
+
+    it "fill outbuf when present" do
+      connection.read_headers!
+      outbuf = String.new
+      buffer = String.new
+      buffer << outbuf while connection.readpartial(2, outbuf)
+      expect(buffer).to eq "1234567890"
+    end
   end
 end