Fix invalid spaces encoding

[tusbar]

MWS doesn’t support + spaces in either query string or POST body, but only %20.

Related: sindresorhus/got#1113

[codecov]

Codecov Report

Merging #158 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #158      +/-   ##
==========================================
+ Coverage   91.97%   92.00%   +0.03%     
==========================================
  Files         183      183              
  Lines        1233     1238       +5     
==========================================
+ Hits         1134     1139       +5     
  Misses         99       99              

Impacted Files	Coverage Δ
lib/client/index.js	98.14% <100.00%> (+0.07%)	⬆️
lib/client/sign.js	100.00% <100.00%> (ø)
lib/client/utils.js	100.00% <100.00%> (ø)

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