Pass MaxMultipartMemory when FormFile is called

[isgj]

When gin.Context.FormFile("...") is called the engine.MaxMultipartMemory is never used. This PR makes sure that the MaxMultipartMemory is passed and removes 2 calls to http.Request.ParseForm since they are called from http.Request.ParseMultipartForm

[codecov]

Codecov Report

Merging #1600 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1600      +/-   ##
==========================================
+ Coverage   99.27%   99.27%   +<.01%     
==========================================
  Files          39       39              
  Lines        1928     1929       +1     
==========================================
+ Hits         1914     1915       +1     
  Misses         10       10              
  Partials        4        4

Impacted Files	Coverage Δ
context.go	99.49% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a1a3256...944f9df. Read the comment docs.

[thinkerou]

@isgj please add some test, thanks!

[isgj]

There was already a test for the happy path. I wrote a test when it fails to check that it returns the error. I you think that another type of test can be written just let me know.

[thinkerou]

LGTM

[thinkerou]

@appleboy please help review, thanks!