Interpolation add missing reserved key

[tjoyal]

Add :exception_handler to the RESERVED_KEYS constant.

Based on https://github.com/svenfuchs/i18n/blob/master/lib/i18n/tests/interpolation.rb

# If no interpolation parameter is not given, I18n should not alter the string.
# This behavior is due to three reasons:
#
#   * Checking interpolation keys in all strings hits performance, badly;
#
#   * This allows us to retrieve untouched values through I18n. For example
#     I could have a middleware that returns I18n lookup results in JSON
#     to be processed through Javascript. Leaving the keys untouched allows
#     the interpolation to happen at the javascript level;
#
#   * Security concerns: if I allow users to translate a web site, they can
#     insert %{} in messages causing the I18n lookup to fail in every request.
#

Passing in a non-interpolation related custom exception handler for a translation would trip the interpolation behaviour when it is unexpected.

It took me a while to understand what was happening, I would suggest to rename the constants toward "interpolation" or move them in lib/i18n/interpolate/ruby.rb.

[radar]

Thank you! I will look at merging this tomorrow when I am back at work.

[radar]

@tjoyal Thanks again :) I think moving the constants closer to where they're used would be the right approach. I'd happily review another PR for that.