Fix #667: Don't buffer output stream when no progress callback is given

[jsyeo]

Description

This fixes the slowness described in #667 by checking if a progress callback is set. If no progress callbacks are given, the outputstream will not be buffered.

Type of change

Check all that apply

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (a change which changes the current internal or external interface)
• This change requires a documentation update

How Has This Been Tested?

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation, if necessary
• My changes generate no new compiler warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Inspect the bytecode viewer, including reasoning why

[codecov]

Codecov Report

Merging #668 into master will decrease coverage by 0.05%.
The diff coverage is 83.33%.

@@             Coverage Diff              @@
##             master     #668      +/-   ##
============================================
- Coverage     71.44%   71.39%   -0.06%     
  Complexity      296      296              
============================================
  Files            58       58              
  Lines          1492     1496       +4     
  Branches        192      193       +1     
============================================
+ Hits           1066     1068       +2     
  Misses          334      334              
- Partials         92       94       +2

Impacted Files	Coverage Δ	Complexity Δ
.../kotlin/com/github/kittinunf/fuel/core/Progress.kt	50% <100%> (+5.55%)	4 <1> (+1)	⬆️
...in/com/github/kittinunf/fuel/toolbox/HttpClient.kt	75.67% <80%> (-1.18%)	28 <0> (-1)

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