speed up accept header parsing a bit.

Accept header is taken from what Safari on El Capitan sends:

```ruby
require 'benchmark/ips'
require 'action_dispatch/http/mime_type'
require 'active_support/all'

accept = 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8'

Benchmark.ips do |x|
  x.report "omg" do
    Mime::Type.parse(accept)
  end
end
```

Before:

```
[aaron@TC actionpack (master)]$ be ruby ../x.rb
Calculating -------------------------------------
                 omg     3.181k i/100ms
-------------------------------------------------
                 omg     35.062k (±12.8%) i/s -    174.955k
[aaron@TC actionpack (master)]$ be ruby ../x.rb
Calculating -------------------------------------
                 omg     3.153k i/100ms
-------------------------------------------------
                 omg     33.724k (±12.4%) i/s -    167.109k
[aaron@TC actionpack (master)]$ be ruby ../x.rb
Calculating -------------------------------------
                 omg     3.575k i/100ms
-------------------------------------------------
                 omg     37.251k (±10.4%) i/s -    185.900k
```

After:

```
[aaron@TC actionpack (master)]$ be ruby ../x.rb
Calculating -------------------------------------
                 omg     3.365k i/100ms
-------------------------------------------------
                 omg     40.069k (±16.1%) i/s -    198.535k
[aaron@TC actionpack (master)]$ be ruby ../x.rb
Calculating -------------------------------------
                 omg     4.168k i/100ms
-------------------------------------------------
                 omg     47.596k (± 7.7%) i/s -    237.576k
[aaron@TC actionpack (master)]$ be ruby ../x.rb
Calculating -------------------------------------
                 omg     4.282k i/100ms
-------------------------------------------------
                 omg     43.626k (±17.7%) i/s -    209.818k
```

1 files changed, 11 insertions, 8 deletions

diff --git a/actionpack/lib/action_dispatch/http/mime_type.rb b/actionpack/lib/action_dispatch/http/mime_type.rb
index 065d7dd355..4672ea7199 100644
--- a/actionpack/lib/action_dispatch/http/mime_type.rb
+++ b/actionpack/lib/action_dispatch/http/mime_type.rb
@@ -1,3 +1,5 @@
+# -*- frozen-string-literal: true -*-
+
 require 'singleton'
 require 'active_support/core_ext/module/attribute_accessors'
 require 'active_support/core_ext/string/starts_ends_with'
@@ -157,7 +159,7 @@ module Mime
     end
 
     class << self
-      TRAILING_STAR_REGEXP = /(text|application)\/\*/
+      TRAILING_STAR_REGEXP = /^(text|application)\/\*/
       PARAMETER_SEPARATOR_REGEXP = /;\s*\w+="?\w+"?/
 
       def register_callback(&block)
@@ -200,15 +202,16 @@ module Mime
           list, index = [], 0
           accept_header.split(',').each do |header|
             params, q = header.split(PARAMETER_SEPARATOR_REGEXP)
-            if params.present?
-              params.strip!
 
-              params = parse_trailing_star(params) || [params]
+            next unless params
+            params.strip!
+            next if params.empty?
+
+            params = parse_trailing_star(params) || [params]
 
-              params.each do |m|
-                list << AcceptItem.new(index, m.to_s, q)
-                index += 1
-              end
+            params.each do |m|
+              list << AcceptItem.new(index, m.to_s, q)
+              index += 1
             end
           end
           AcceptList.sort! list