Preserve quoted parameter values when parsing and serializing

[OskarEichler]

Summary

Preserve escaped backslashes and quotes, and serialize empty strings as quoted values. Previously values could be corrupted or serialized into text the parser rejected.

Reproduction and verification

p = WebSocket::Extensions::Parser
value = 'a' + 92.chr + 'b'
header = p.serialize_params('ext', {'v' => value})
p p.parse_header(header).by_name('ext').first['v'] == value # true

Eleven focused cases and 2,000 generated quoted-value round trips pass. Existing broadly accepted quoted-string values remain supported; this does not claim every such value satisfies a particular WebSocket extension specification.

• Ruby 4.0.6 through rbenv; existing current-upstream suite: 64 examples, zero failures on this isolated change.
• The cumulative release-based candidate passes 283 focused checks (272 baseline failures → zero), 2,784 model checks, the current upstream's 64-example suite, and gem build/extraction.
• The historical 0.1.5 suite has three Ruby keyword-versus-options-hash mock failures on both baseline and candidate. Current upstream already corrected those expectations; they were run against the candidate through an external preload without changing repository tests.
• No new or modified tests/specs, following the consumer repository's explicit policy. Reproductions/models were run from external scratch scripts.

Breaking changes and limitations

No intended breaking change. Backslashes/empty values round-trip correctly; numeric conversion behavior remains unchanged.

Only local Ruby 4.0.6/macOS execution is claimed; the repository's older Ruby/JRuby matrix needs upstream CI. No production access or unrelated release upgrades.