[ruby/net-http] use +'' everywhere

https://github.com/ruby/net-http/commit/1077427152

lib/net/http/generic_request.rb

@@ -316,7 +316,7 @@ class Net::HTTPGenericRequest
     boundary ||= SecureRandom.urlsafe_base64(40)
     chunked_p = chunked?
 
-    buf = String.new
+    buf = +''
     params.each do |key, value, h={}|
       key = quote_string(key, charset)
       filename =
@@ -401,7 +401,7 @@ class Net::HTTPGenericRequest
     if /[\r\n]/ =~ reqline
       raise ArgumentError, "A Request-Line must not contain CR or LF"
     end
-    buf = String.new
+    buf = +''
     buf << reqline << "\r\n"
     each_capitalized do |k,v|
       buf << "#{k}: #{v}\r\n"

lib/net/http/response.rb

@@ -648,7 +648,7 @@ class Net::HTTPResponse
     if block
       Net::ReadAdapter.new(block)
     else
-      dest || String.new
+      dest || +''
     end
   end