Hot fix: allow writing data of arbitrary size during hijack

[hwh33]

Fixing a bug introduced by https://github.com/getlantern/tlsmasq/pull/12. Just going to merge this in.

• [ x ] Do the tests pass? Consistently?
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• [ x ] Is the code in question being linted? If not, consider adding a linter step to CI. If yes, make sure the linter is happy.
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• [ x ] Did you create new documentation? Is it accessible and in the right place?
• [ x ] Has existing documentation been updated?
• [ x ] Have you logged tickets for related technical debt with the label “techdebt”?