capitalize() keeps full uppercase strings instead of normalizing case

[OscarHenry]

Description

In the latest version of stringr, the capitalize() extension behaves unexpectedly when the input string is already in full uppercase.

Example:

"HELLO".capitalize()

Current output:

HELLO

Expected output:

Hello

The method currently uppercases the first character but leaves the rest of the string unchanged. This causes fully-uppercase inputs to remain fully uppercase, which defeats the purpose of a typical capitalize operation (first letter uppercase, remaining letters lowercase).

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Root cause

In case.dart, the current implementation is:

false => isEmpty ? '' : this[0].toUpperCase() + substring(1),

Here, substring(1) preserves the original casing of the remainder of the string.

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Proposed fix

Lowercase the remainder explicitly:

false => isEmpty
    ? ''
    : this[0].toUpperCase() + substring(1).toLowerCase(),

This produces consistent behavior:

"HELLO".capitalize() // Hello
"hello".capitalize() // Hello
"HeLLo".capitalize() // Hello

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Why this matters

Most capitalize implementations across ecosystems normalize the string to:

Uppercase first letter + lowercase remainder

Current behavior is surprising and inconsistent, especially when working with API responses or legacy data that may arrive in all caps.

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Happy to submit a PR if this approach is acceptable 👍