only decompress with proper content-encoding header

[jerrm]

Closes #437

[codecov-io]

Codecov Report

Merging #438 (001a345) into master (b304eef) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #438      +/-   ##
============================================
- Coverage     93.12%   93.07%   -0.05%     
- Complexity      763      764       +1     
============================================
  Files            21       21              
  Lines          1788     1790       +2     
============================================
+ Hits           1665     1666       +1     
- Misses          123      124       +1     

Impacted Files	Coverage Δ	Complexity Δ
library/Requests.php	81.78% <100.00%> (+0.11%)	120.00 <0.00> (+1.00)
library/Requests/Transport/fsockopen.php	94.35% <0.00%> (-0.57%)	69.00% <0.00%> (ø%)

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[jrfnl]

@jerrm Hiya!

We're currently planning two triage sessions for Requests 2.0 and would like to invite you to join us in one or both of these sessions to talk us through the PR and discuss it.

The triage sessions are currently planned for:

• Friday August 20, 07:00 - 11:00 UTC
• Friday September 3, 07:00 - 11:00 UTC

Would you be available to join us during those times on one of those days ?

Also: would you be willing and able to add some tests for this change to the PR ?

[jrfnl]

Some notes:

• Needs rebase & conflict solving.
• The hard-coded array should become a class constant.
• Check needs to be done whether the array is complete for these purposes.
• Can this be done without using the performance-poor array_intersect() ?
• Needs tests.