fix(method-override): discard stale form content length - #5521

Removing the override field from a URLencoded form changes its body length, but the rewritten request keeps the incoming Content-Length. For _method=DELETE&message=Hello, the body shrinks from 28 to 13 bytes. Forwarding it through Node's fetch then fails with UND_ERR_REQ_CONTENT_LENGTH_MISMATCH.

Drop the stale header when rebuilding the form request, matching the existing multipart branch. Keep the content type and other headers. Requests without a method override retain their original length.

Fixes #5520

Validation:

• New regressions fail on unchanged code for both default and custom form fields; the unchanged-request control passes. All 15 method-override tests pass with the fix.
• A real local HTTP forwarding reproduction fails before the fix and returns 200 with the correct body and length afterward.
• pnpm run test: 5,199 passed, 44 skipped, including TypeScript checks. NO_COLOR was unset for the color tests.
• pnpm run test:node: 14 passed. pnpm run test:bun: 16 passed.
• pnpm run build, pnpm run check:fix, and pnpm run editorconfig-checker: passed.

The author should do the following, if applicable

• Add tests
• Run tests
• pnpm run check:fix to format and lint the code
• Add TSDoc/JSDoc to document code (no API change)