Fix inefficient assert pattern in Rack::Lint

Partial backport of 1970771

This fixes an issue encountered when adding `Rack::Lint` to the Rails
test suite. Rails puts a lazily evaluated class inside the request env
that has the potential to raise when converted to a string. Since this
assertion in `Rack::Lint` calls `#inspect` on `env` whether or not `env`
is a `Hash`, the lazily evaluated class would raise at that point and
prevent the rest of `Rack::Lint` from validating the conformity of the
request.

By backporting this change, `#inspect` is now only called when the
`LintError` would be raised which avoids the problem.

Co-authored-by: Benoit Daloze <eregontp@gmail.com>

lib/rack/lint.rb

@@ -78,9 +78,7 @@ def check_env(env)
       ## The environment must be an unfrozen instance of Hash that includes
       ## CGI-like headers.  The application is free to modify the
       ## environment.
-      assert("env #{env.inspect} is not a Hash, but #{env.class}") {
-        env.kind_of? Hash
-      }
+      raise LintError, "env #{env.inspect} is not a Hash, but #{env.class}" unless env.kind_of? Hash
       assert("env should not be frozen, but is") {
         !env.frozen?
       }