Check envelope size before sending it

[st0012]

Because we now need to check if a serialized envelope item is oversized (see #1603 (comment)), envelope serialization is better performed by Transport to have more control to filter oversized items.

[st0012]

@sl0thentr0py I know the code could still be refactored (which I'll do later), but can you give it a quick scan and see if the approach looks good?

[sl0thentr0py]

logic looks ok 👍

[codecov-commenter]

Codecov Report

Merging #1747 (23652c9) into master (e727167) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1747      +/-   ##
==========================================
+ Coverage   98.43%   98.45%   +0.01%     
==========================================
  Files         144      144              
  Lines        8312     8412     +100     
==========================================
+ Hits         8182     8282     +100     
  Misses        130      130              

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/envelope.rb	95.23% <ø> (-4.77%)	⬇️
sentry-ruby/lib/sentry/event.rb	98.90% <100.00%> (+0.01%)	⬆️
sentry-ruby/lib/sentry/transport.rb	99.08% <100.00%> (+0.19%)	⬆️
...-ruby/spec/sentry/transport/http_transport_spec.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry/transport_spec.rb	100.00% <100.00%> (ø)
sentry-resque/lib/sentry/resque.rb	100.00% <0.00%> (+2.85%)	⬆️

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