Respect socket timeout

[gabrielfalcao]

This PR closes #430.

@mariojonke thanks for the test case, I'll mention it in a new section of "guides" in the docs.

Your test-case caught a mistake I made in fixing #428 😅 :

HTTPretty/httpretty/core.py

Line 700 in 19880d8

t.join(None)

[codecov]

Codecov Report

Merging #431 (da624d4) into master (19880d8) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #431   +/-   ##
=======================================
  Coverage   81.94%   81.94%           
=======================================
  Files           7        7           
  Lines        1113     1113           
  Branches      159      159           
=======================================
  Hits          912      912           
  Misses        147      147           
  Partials       54       54           

Impacted Files	Coverage Δ
httpretty/core.py	80.51% <100.00%> (ø)

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