Using body buffer only if writing to it is finished

[eyalkoren]

What does this PR do?

(hopefully) fixes #1566

Checklist

• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix

[codecov-io]

Codecov Report

Merging #1579 (0447308) into master (bc6193f) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1579      +/-   ##
============================================
+ Coverage     58.66%   58.68%   +0.02%     
  Complexity       92       92              
============================================
  Files           396      396              
  Lines         17795    17797       +2     
  Branches       2468     2468              
============================================
+ Hits          10439    10445       +6     
+ Misses         6642     6639       -3     
+ Partials        714      713       -1     

Impacted Files	Coverage Δ	Complexity Δ
...ava/co/elastic/apm/agent/impl/context/Request.java	86.00% <100.00%> (+4.36%)	0.00 <0.00> (ø)
.../apm/agent/report/serialize/DslJsonSerializer.java	88.47% <100.00%> (ø)	0.00 <0.00> (ø)
...ic/apm/agent/profiler/collections/LongHashSet.java	18.25% <0.00%> (-1.20%)	0.00% <0.00%> (ø%)
...tic/apm/agent/report/ssl/TLSFallbackSSLSocket.java	41.30% <0.00%> (+2.17%)	0.00% <0.00%> (ø%)

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Build stats

• Build Cause: Pull request #1579 updated

• Start Time: 2020-12-27T16:18:14.409+0000

• Duration: 45 min 15 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1680
Skipped	12
Total	1692