Replace http methods to constants

[shoman4eg]

Replace all http methods from string to http constants
Replace all http status codes to contants

[codecov]

Codecov Report

Merging #317 (015a18c) into master (1c85553) will not change coverage.
The diff coverage is 95.45%.

@@           Coverage Diff           @@
##           master     #317   +/-   ##
=======================================
  Coverage   74.42%   74.42%           
=======================================
  Files          57       57           
  Lines        5446     5446           
=======================================
  Hits         4053     4053           
  Misses       1022     1022           
  Partials      371      371           

Impacted Files	Coverage Δ
v2/futures/order_service.go	59.04% <70.00%> (ø)
v2/margin_service.go	69.76% <85.00%> (ø)
v2/account_service.go	82.22% <100.00%> (ø)
v2/asset_detail_service.go	50.00% <100.00%> (ø)
v2/asset_dividend_service.go	82.85% <100.00%> (ø)
v2/client.go	85.04% <100.00%> (ø)
v2/delivery/account_service.go	69.23% <100.00%> (ø)
v2/delivery/client.go	83.10% <100.00%> (ø)
v2/delivery/exchange_info_service.go	86.95% <100.00%> (ø)
v2/delivery/kline_service.go	86.79% <100.00%> (ø)
... and 35 more

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