Modified guessDefaultEscapingStrategy to not escape txt templates

Useful for plaintext emails.
bug #9639 Modified guessDefaultEscapingStrategy to not escape txt templates (fabpot)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #9639).

Discussion
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Modified guessDefaultEscapingStrategy to not escape txt templates

This PR was submitted on the symfony/TwigBundle read-only repository and moved automatically to the main Symfony repository (closes symfony/TwigBundle#2).

Useful for plaintext emails.

Commits
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6affe94 Modified guessDefaultEscapingStrategy to not escape txt templates