Make ContentHandlers a public member of the EncodeDecodeMiddleware

[dselivanov]

• makes ContentHandlers a public member of the EncodeDecodeMiddleware
• changes class of the HTTPError: 'HTTPErrorFactory' -> 'HTTPError'
• changes class of the ContentHandlers: 'ContentHandler' -> 'ContentHandlers'
• adjusts tests and docs accordingly

[codecov[bot]]

Codecov Report

Merging #119 into dev will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #119      +/-   ##
==========================================
+ Coverage   89.78%   89.79%   +<.01%     
==========================================
  Files          26       26              
  Lines        1302     1303       +1     
==========================================
+ Hits         1169     1170       +1     
  Misses        133      133

Impacted Files	Coverage Δ
R/Application.R	98.06% <ø> (ø)	:arrow_up:
R/HTTPError.R	100% <ø> (ø)	:arrow_up:
R/EncodeDecodeMiddleware.R	96.51% <100%> (ø)

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