v1.2.0

Adds a ghtest sub-package with two helpers that hide the non-discoverable
GitHub-specific traps in testing ghkit-using code: secondary-rate-limit
classification (the documentation_url suffix gofri/go-github-ratelimit
pattern-matches on) and the bored-engineer ETag hash domain (which includes
the request Authorization header). Plus a TESTING.md with self-contained
recipes for the surrounding stdlib bits. No new runtime dependencies.

Added

ghtest sub-package

• ghtest.WriteSecondaryLimit(w, retryAfter) writes a 403 with Retry-After
  (whole seconds; negative durations clamp to zero) and a JSON body whose
  documentation_url ends in #secondary-rate-limits. That suffix is what
  gofri/go-github-ratelimit pattern-matches on to classify the response as
  an AbuseRateLimitError, so consumer retry paths actually trigger in
  tests instead of silently falling through.
• ghtest.Write304IfMatch(w, r, body) bool computes the expected ETag using
  the bored-engineer algorithm (hashes Authorization, Accept, and Cookie
  request headers along with the body), normalises every tag in
  If-None-Match (split on commas, trim whitespace, strip W/ weak prefix
  and surrounding quotes), and on any match sets a quoted RFC 7232 ETag
  response header, writes 304 Not Modified with empty body, and returns
  true. Returns false and writes nothing on miss.
• Both helpers compose with stdlib httptest.NewServer; consumers bring
  their own handler. Tests cover quoted/unquoted/weak-only matches, multi-tag
  comma splitting, no-match leaves response untouched, no-header
  short-circuit, and different-body misses.

Documentation

• New TESTING.md with self-contained recipes: routing a ghkit-built client
  at a test server (the gh.BaseURL = url.Parse(srv.URL+"/") pattern),
  ETag 304 replay handling, rate-limit header emission (no helper, recipe
  only), Link-header pagination (no helper, recipe only), secondary-rate-
  limit testing, plus a "See also" pointer to migueleliasweb/go-github-mock
  for the SDK-layer mocking case.
• README "Testing your code" section, three lines linking to TESTING.md.

Dependencies

No changes. Same as 1.1.0.