Refactor multipart body parser

[artemklevtsov]

Small refactor to simplify code.

[codecov[bot]]

Codecov Report

Merging #153 into dev will increase coverage by 0.07%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #153      +/-   ##
==========================================
+ Coverage   94.90%   94.98%   +0.07%     
==========================================
  Files          27       27              
  Lines        1275     1275              
==========================================
+ Hits         1210     1211       +1     
+ Misses         65       64       -1     

Impacted Files	Coverage Δ
R/Application.R	96.71% <ø> (ø)
src/parse_multipart.cpp	98.78% <100.00%> (+1.16%)	:arrow_up:
R/Request.R	89.47% <0.00%> (-0.10%)	:arrow_down:
R/Router.R	98.80% <0.00%> (+0.04%)	:arrow_up:

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[lintr-bot]

R/Router.R:168:9: style: Place a space before left parenthesis, except in a function call.

​      if(identical(prefix, "")) {
        ^