clear http response inputstream and errorstream to reuse connection.

[yllions]

I missed some cases in the previous pull request. It is when server status code is more than 400, you will get an excepion if you try to open input stream. I have fixed it in this pull request.

1. you should test scenarios when server response code is more than 400. In my test, this patch can dramatically decrease connections.
2. In normal cases, you will get null if you try to get error inputstream and it is ok.

[coveralls]

Changes Unknown when pulling 8d31e51 on xiaoerlyl:keepalive into ** on ctripcorp:master**.

[codecov-io]

Codecov Report

Merging #863 into master will increase coverage by 0.03%.
The diff coverage is 84.61%.

@@             Coverage Diff              @@
##             master     #863      +/-   ##
============================================
+ Coverage      46.9%   46.93%   +0.03%     
  Complexity     1544     1544              
============================================
  Files           352      352              
  Lines          9703     9715      +12     
  Branches        961      962       +1     
============================================
+ Hits           4551     4560       +9     
- Misses         4811     4812       +1     
- Partials        341      343       +2

Impacted Files	Coverage Δ	Complexity Δ
...com/ctrip/framework/apollo/util/http/HttpUtil.java	85.71% <84.61%> (-0.78%)	7 <0> (+1)
...ervice/service/ReleaseMessageServiceWithCache.java	84.52% <0%> (-1.2%)	23% <0%> (-1%)

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[nobodyiam]

Thanks for your detailed explanation and fix!

In Apollo cases, the normal response code will only be 200 and 304, so is it appropriate to open isr when status code is 200 and 304, and open esr otherwise?

The code may look a little bit cleaner in that way. But I have not tested yet.

[coveralls]

Coverage increased (+0.05%) to 50.468% when pulling b2f24cb on xiaoerlyl:keepalive into d0bbf0d on ctripcorp:master.

[nobodyiam]

Thanks, I just tested the scenarios you mentioned, and it does work!