Add ability to provide regexp instead of just substrings for exclude

This is another go at eparreno#23,

Where we make it a little more flexible by allowing either the old
string version or a now newly introduced regxp.

This is fully backwards compatible.

lib/rack/jwt/auth.rb

@@ -152,22 +152,31 @@ def check_exclude_type!
         end
 
         @exclude.each do |x|
-          unless x.is_a?(String)
-            raise ArgumentError, 'each exclude Array element must be a String'
+          unless x.is_a?(String) || x.is_a?(Regexp)
+            raise ArgumentError, 'each exclude Array element must be a String or a Regexp'
           end
 
-          if x.empty?
+          if x.to_s.empty?
             raise ArgumentError, 'each exclude Array element must not be empty'
           end
 
-          unless x.start_with?('/')
+          # Perhaps surprisingly, Regexp#inspect actually produces the more
+          # natural version of the string than #to_s.
+          as_s = x.is_a?(Regexp) ? x.inspect : x
+          unless as_s.start_with?('/')
             raise ArgumentError, 'each exclude Array element must start with a /'
           end
         end
       end
 
       def path_matches_excluded_path?(env)
-        @exclude.any? { |ex| env['PATH_INFO'].start_with?(ex) }
+        @exclude.any? do |ex|
+          if ex.is_a?(Regexp)
+            ex.match?(env['PATH_INFO'])
+          else
+            env['PATH_INFO'].start_with?(ex)
+          end
+        end
       end
 
       def valid_auth_header?(env)

spec/exclusion_spec.rb

@@ -22,6 +22,15 @@
       end
     end
 
+    describe 'passes through matching regexp' do
+      let(:app) { Rack::JWT::Auth.new(inner_app, secret: secret, exclude: [/stati+/]) }
+
+      it 'returns a 200' do
+        get('/static')
+        expect(last_response.status).to eq 200
+      end
+    end
+
     describe 'passes through matching exact path with trailing slash' do
       let(:app) { Rack::JWT::Auth.new(inner_app, secret: secret, exclude: ['/static']) }