fix: get body size 0 (#300)

[shiguangyin]

when the http status code is 307 or 308, the request.Body will be closed, after that the func GetBody will be called but the bodyBuf was closed, then we will get an error http: ContentLength=xxx with Body length 0

[shiguangyin]

see https://github.com/golang/go/blob/72a9dec156408a87548deb920a67b8bf787062db/src/net/http/client.go#L737

[codecov]

Codecov Report

Merging #380 into master will decrease coverage by 0.69%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #380      +/-   ##
==========================================
- Coverage   96.20%   95.51%   -0.70%     
==========================================
  Files          10       10              
  Lines        1238     1003     -235     
==========================================
- Hits         1191      958     -233     
+ Misses         26       25       -1     
+ Partials       21       20       -1     

Impacted Files	Coverage Δ
client.go	95.78% <ø> (-0.95%)	⬇️
middleware.go	91.42% <100.00%> (-0.12%)	⬇️
redirect.go	92.00% <0.00%> (-2.12%)	⬇️
util.go	91.93% <0.00%> (-2.01%)	⬇️
retry.go	97.01% <0.00%> (-0.36%)	⬇️
resty.go	100.00% <0.00%> (ø)
trace.go	100.00% <0.00%> (ø)
request.go	100.00% <0.00%> (ø)
... and 2 more

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[jeevatkm]

@shiguangyin Thanks for the PR to fix the body size issue on redirects.

Can you please improve unit test cases for missed lines? you can see the code coverage report here #380 (comment)

[shiguangyin]

@jeevatkm I think the code coverage report is not correct, I don't know why the code coverage decreased, I did't have any change on some files like redirect.go and util.go, but the report show those files are impacted. I don't know how to improve the unit test cases to increase the code coverage