private: Add '%' character to GST_ASCII_IS_STRING

HTML-escaped string can have '%'. If GST_ASCII_IS_STRING
allows '%', we can use the string without a backslash.

Fixes: #57
18 jobs for fixes/57 in 54 minutes and 55 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #326302
gst indent

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passed #326301
manifest

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  Build
passed #326303
build fedora x86_64

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passed #326304
build nodebug fedora x86_64

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  Test
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check fedora

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integration testsuites fedora

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  Full Builds
passed #326308
build cerbero android universal

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passed #326309
build cerbero cross win32

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passed #326310
build cerbero cross win64

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passed #326307
build cerbero fedora x86_64

00:07:24

 
  Full Tests
passed #326315
valgrind bad

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passed #326312
valgrind base

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passed #326311
valgrind core

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passed #326316
valgrind ges

00:05:00

failed #326313
valgrind good

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passed #326314
valgrind ugly

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  Apps
skipped #326317
android universal examples
 
  Deploy
skipped #326318
documentation
 
Name Stage Failure
failed
valgrind good Full Tests
           Total: 643
Uploading artifacts...
gst-build/build/meson-logs/: found 2 matching files
validate-logs: found 1384 matching files

Uploading artifacts to coordinator... ok
id=326313 responseStatus=201 Created token=nuPT1qQb
Uploading artifacts...
validate-logs/*.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=326313 responseStatus=201 Created token=nuPT1qQb
ERROR: Job failed: exit code 1