Add verification of the boundary

[christopher-dG]

As specified here: https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html

boundary := 0*69<bchars> bcharsnospace 

bchars := bcharsnospace / " " 

bcharsnospace :=    DIGIT / ALPHA / "'" / "(" / ")" / "+"  / 
"_" 
               / "," / "-" / "." / "/" / ":" / "=" / "?" 

[codecov-io]

Codecov Report

Merging #615 into master will increase coverage by 0.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #615      +/-   ##
==========================================
+ Coverage   76.27%   76.57%   +0.29%     
==========================================
  Files          37       37              
  Lines        2268     2271       +3     
==========================================
+ Hits         1730     1739       +9     
+ Misses        538      532       -6     

Impacted Files	Coverage Δ
src/multipart.jl	33.33% <100.00%> (+2.56%)	⬆️
src/ConnectionPool.jl	72.59% <0.00%> (-0.36%)	⬇️
src/debug.jl	63.33% <0.00%> (+23.33%)	⬆️

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[quinnj]

Thanks for doing this!