Avoid double JSON parse for metadata-less messages

In rails#46612, a check was added to only attempt metadata extraction if the
message looks like a JSON object (i.e. starts with "{"), thus avoiding
an unnecessary JSON parse and possible exception.

This commit extends the check to only attempt metadata extraction if the
message looks like a metadata wrapper object (i.e. has the "_rails"
key).  This avoids an unnecessary JSON parse of JSON object messages
that don't have metadata.

**Benchmark**

  ```ruby
  require "benchmark/ips"
  require "active_support/all"

  verifier = ActiveSupport::MessageVerifier.new("secret", serializer: JSON)
  message_100 = verifier.generate({ content: "x" * 100 })
  message_1m = verifier.generate({ content: "x" * 1_000_000 })

  Benchmark.ips do |x|
    x.report("100 chars") do
      verifier.verify(message_100)
    end

    x.report("1m chars") do
      verifier.verify(message_1m)
    end
  end
  ```

**Before**

  ```
  Warming up --------------------------------------
             100 chars     2.803k i/100ms
              1m chars     6.000  i/100ms
  Calculating -------------------------------------
             100 chars     27.762k (± 1.6%) i/s -    140.150k in   5.049649s
              1m chars     83.516  (±16.8%) i/s -    402.000  in   5.037269s
  ```

**After**

  ```
  Warming up --------------------------------------
             100 chars     3.360k i/100ms
              1m chars     9.000  i/100ms
  Calculating -------------------------------------
             100 chars     33.480k (± 1.7%) i/s -    168.000k in   5.019311s
              1m chars    113.373  (±15.0%) i/s -    549.000  in   5.023443s
  ```

Author: jonathanhefner

activesupport/lib/active_support/messages/metadata.rb

@@ -38,11 +38,11 @@ def pick_expiry(expires_at, expires_in)
 
           def extract_metadata(message)
             begin
-              data = JSON.decode(message) if message.start_with?("{")
+              data = JSON.decode(message) if message.start_with?('{"_rails":')
             rescue ::JSON::JSONError
             end
 
-            if data.is_a?(Hash) && data.key?("_rails")
+            if data
               new(decode(data["_rails"]["message"]), data["_rails"]["exp"], data["_rails"]["pur"])
             else
               new(message)