HTML API: Ensure attribute updates happen only once for case variants

[dmsnell]

Github PR: ​#4337

When setting a new value for an attribute multiple times and providing
multiple case variations of the attribute name the Tag Processor has
been appending multiple copies of the attribute into the updated HTML.

This means that only the first attribute set determines the value in
the final output, plus the output will appear wrong.

In this patch we're adding a test to catch the situation and resolving it
by using the appropriate comparable attribute name as a key for storing
the updates as we go. Previously we stored updates to the attribute by
its given $name, but when a new update of the same name with a
case variant was queued, it would not override the previously-enqueued
value as it out to have.

This bug has not been reported or known to be exposed.
Reproduce in trunk by enqueuing multiple updates to a given
attribute with different case variants while visiting a single tag
with the Tag Processor. (See included unit tests).

HTML API: Ensure attribute updates happen only once for case variants

Trac ticket: #58146-trac

When setting a new value for an attribute multiple times and providing
multiple case variations of the attribute name the Tag Processor has
been appending multiple copies of the attribute into the updated HTML.

This means that only the first attribute set determines the value in
the final output.

In this patch we're adding a test to catch the situation, and then fixing
the bug by using the comparable name to key the attribute updates instead
of the case-sensitive name.

Great fix and great catch!

[Bernhard Reiter]

In 55659:

HTML API: Ensure attribute updates happen only once for case variants.

When setting a new value for an attribute multiple times and providing
multiple case variations of the attribute name the Tag Processor has
been appending multiple copies of the attribute into the updated HTML.

This means that only the first attribute set determines the value in
the final output, plus the output will appear wrong.

In this patch we're adding a test to catch the situation and resolving it
by using the appropriate comparable attribute name as a key for storing
the updates as we go. Previously we stored updates to the attribute by
its given $name, but when a new update of the same name with a
case variant was queued, it would not override the previously-enqueued
value as it out to have.

Props dmsnell, zieladam.
Fixes #58146.

Committed to Core trunk in `r55659`.

[Bernhard Reiter]

Reopening for 6.2.1 consideration.

[Bernhard Reiter]

In 55662:

HTML API: Ensure attribute updates happen only once for case variants.

When setting a new value for an attribute multiple times and providing
multiple case variations of the attribute name the Tag Processor has
been appending multiple copies of the attribute into the updated HTML.

This means that only the first attribute set determines the value in
the final output, plus the output will appear wrong.

In this patch we're adding a test to catch the situation and resolving it
by using the appropriate comparable attribute name as a key for storing
the updates as we go. Previously we stored updates to the attribute by
its given $name, but when a new update of the same name with a
case variant was queued, it would not override the previously-enqueued
value as it out to have.

Props dmsnell, zieladam.
Merges [55659] to the 6.2 branch.
Fixes #58146.

Backported to Core's 6.2 branch in `r55662`.