Fix handling of escaped opening brace in format strings

[Flamefire]

When using an escaped opening brace in a format string it was output 2 times instead of one. Fix and add test

Fixes #194

[codecov[bot]]

Codecov Report

Merging #195 (744799a) into develop (e1b20ca) will increase coverage by 0.10%. The diff coverage is 100.00%.

Additional details and impacted files

[](https://app.codecov.io/gh/boostorg/locale/pull/195?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) ```diff @@ Coverage Diff @@ ## develop #195 +/- ## =========================================== + Coverage 94.82% 94.92% +0.10% =========================================== Files 114 114 Lines 9878 9917 +39 =========================================== + Hits 9367 9414 +47 + Misses 511 503 -8 ``` | [Files Changed](https://app.codecov.io/gh/boostorg/locale/pull/195?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) | Coverage Δ | | |---|---|---| | [include/boost/locale/format.hpp](https://app.codecov.io/gh/boostorg/locale/pull/195?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-aW5jbHVkZS9ib29zdC9sb2NhbGUvZm9ybWF0LmhwcA==) | `97.82% <100.00%> (+0.01%)` | :arrow_up: | | [test/test\_formatting.cpp](https://app.codecov.io/gh/boostorg/locale/pull/195?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-dGVzdC90ZXN0X2Zvcm1hdHRpbmcuY3Bw) | `100.00% <100.00%> (ø)` | | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/boostorg/locale/pull/195/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/boostorg/locale/pull/195?src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/boostorg/locale/pull/195?src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Last update [e1b20ca...744799a](https://app.codecov.io/gh/boostorg/locale/pull/195?src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg).