AXIS2-6101 Add unit tests for HC5 5.6+ content encoding behavior

4 tests covering the double-decompression fix:
- testDecompressedEntityWithGzipHeader: HC5 5.6+ scenario where
  Content-Encoding header says "gzip" but entity encoding is null
  (already decompressed). Verifies Axis2 does NOT re-decompress.
- testEntityWithGzipContentEncoding: entity encoding says "gzip"
  (pre-5.6 behavior or decompression disabled). Axis2 should
  decompress.
- testNoContentEncoding: no encoding at all (common case).
- testIdentityContentEncoding: "identity" encoding treated as
  no encoding.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>