check for stale connection while framing/chunking message to send

[tanmaykm]

This fixes a condition where sending a message on a stale connection can potentially get into an infinite loop while chunking a large message.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@d7bc69f). Click here to learn what that means.
The diff coverage is 57.37%.

@@            Coverage Diff            @@
##             master      #46   +/-   ##
=========================================
  Coverage          ?   82.18%           
=========================================
  Files             ?        9           
  Lines             ?     1016           
  Branches          ?        0           
=========================================
  Hits              ?      835           
  Misses            ?      181           
  Partials          ?        0           

Impacted Files	Coverage Δ
src/amqps.jl	0.00% <0.00%> (ø)
src/buffered_socket.jl	0.00% <0.00%> (ø)
src/spec.jl	0.00% <ø> (ø)
src/protocol.jl	87.03% <81.25%> (ø)
src/convert.jl	85.71% <83.33%> (ø)
src/types.jl	96.91% <83.33%> (ø)
src/auth.jl	100.00% <100.00%> (ø)
src/message.jl	91.66% <100.00%> (ø)

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