middleware: add ReserveRequest and ReserveResponseWriter

[veezhang]

• support reserve http.Request
• support reserve http.ResponseWriter
• with 100% test coverage

In some cases, we need to use http.Request and http.ResponseWriter in our business code.
For examples:

• file download
• Parse the request yourself (preferably using framework parse)

[codecov-commenter]

Codecov Report

Merging #18 (b71aaf6) into master (97328dd) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master       #18    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           13        16     +3     
  Lines          624       769   +145     
==========================================
+ Hits           624       769   +145     

Impacted Files	Coverage Δ
middleware/assets.go	100.00% <100.00%> (ø)
middleware/keep_request_responce.go	100.00% <100.00%> (ø)
middleware/middleware.go	100.00% <100.00%> (ø)

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[nianiaJR]

LGTM