Extend coverage

[filipecosta90]

• Increases coverage on document and spellcheck. We should now be at ~70%.
• extended Paging tests to make sure we're OK on serialization.
• reduces complexity of FT.AGGREGATE response parsing.
• extends FT.AGGREGATE tests to make sure we're correctly processing replies.

[codecov]

Codecov Report

Merging #48 into master will increase coverage by 4.57%.
The diff coverage is 79.31%.

@@            Coverage Diff            @@
##           master     #48      +/-   ##
=========================================
+ Coverage   63.53%   68.1%   +4.57%     
=========================================
  Files          12      12              
  Lines         905     925      +20     
=========================================
+ Hits          575     630      +55     
+ Misses        272     239      -33     
+ Partials       58      56       -2

Impacted Files	Coverage Δ
redisearch/client.go	67.45% <50%> (+0.2%)	⬆️
redisearch/aggregate.go	69.69% <81.25%> (+0.82%)	⬆️
redisearch/spellcheck.go	86.84% <81.81%> (+38.35%)	⬆️
redisearch/document.go	72.72% <0%> (+36.36%)	⬆️

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[filipecosta90]

@gkorland can you review this one. No added feature, just increased coverage :)

[lgtm-com]

This pull request introduces 1 alert when merging 5cfa76b into e440655 - view on LGTM.com

new alerts:

• 1 for Useless assignment to local variable

[itamarhaber]

LGTM