fix(listener): avoid mutating caller header object when setting Content-Length

[agustin18]

Fixes #400

Description

In responseViaCache, when header is a plain object and no Content-Length was specified, header['Content-Length'] = ... mutated the caller's header object in place.

If that header object is reused across responses (e.g., a module-level constant) or frozen via Object.freeze(...):

1. On reused plain headers, subsequent requests trigger a TypeError: v is not iterable (HTTP 500) because Hono's response handler iterates non-string values as multi-value headers.
2. On frozen headers, direct mutation throws TypeError: Cannot add property Content-Length, object is not extensible (HTTP 500).

Solution

Instead of mutating the incoming header object in place, create a shallow copy with Content-Length only when it needs to be injected:

if (contentLength !== undefined) {
  header = {
    ...header,
    'Content-Length': contentLength,
  }
}

Tests

• Added parameterized tests covering string, Uint8Array, and Blob bodies to ensure caller header objects remain unmodified across sequential requests.
• Added test verifying frozen header objects (Object.freeze) do not cause runtime errors.
• Verified all 439 tests pass cleanly.