Merging @mzsanford pull request (Thank you!)

Squeezing the force_mime_type method into a *sub!* call

Improving code readability

Making the helper methods and constants *protected*

lib/rack/jsonp.rb

@@ -1,8 +1,6 @@
 module Rack
 
   class JSONP
-    # Do not allow arbitrary Javascript in the callback.
-    VALID_CALLBACK_PATTERN = /^[a-zA-Z0-9\._]+$/
 
     def initialize(app)
       @app = app
@@ -13,7 +11,7 @@ def call(env)
       requesting_jsonp = Pathname(request.env['PATH_INFO']).extname =~ /^\.jsonp$/i
       callback = request.params['callback']
 
-      return [400,{},[]] if requesting_jsonp && (!callback || !callback.match(VALID_CALLBACK_PATTERN))
+      return [400,{},[]] if requesting_jsonp && !self.valid_callback?(callback)
 
       if requesting_jsonp
         env['PATH_INFO'].sub!(/\.jsonp/i, '.json')
@@ -22,21 +20,41 @@ def call(env)
 
       status, headers, body = @app.call(env)
 
-      if requesting_jsonp && headers['Content-Type'] && headers['Content-Type'].match(/application\/json/i)
+      if requesting_jsonp && self.json_response?(headers['Content-Type'])
         json = ""
         body.each { |s| json << s }
         body = ["#{callback}(#{json});"]
         headers['Content-Length'] = Rack::Utils.bytesize(body[0]).to_s
-        headers['Content-Type'] = force_mime_type(headers['Content-Type'], 'application/javascript')
+        headers['Content-Type'].sub!(/^[^;]+(;?)/, "#{MIME_TYPE}\\1")
       end
 
       [status, headers, body]
     end
 
-    def force_mime_type(content_type, mime_type)
-      content_type_parts = (content_type || '').split(/;/)
-      content_type_parts[0] = mime_type
-      content_type_parts.join(';')
+  protected
+
+    # Do not allow arbitrary Javascript in the callback.
+    #
+    # @return [Regexp]
+    VALID_CALLBACK_PATTERN = /^[a-zA-Z0-9\._]+$/
+
+    # @return [String] the JSONP response mime type.
+    MIME_TYPE = 'application/javascript'
+
+    # Checks if the callback function name is safe/valid.
+    #
+    # @param [String] callback the string to be used as the JSONP callback function name.
+    # @return [TrueClass|FalseClass]
+    def valid_callback?(callback)
+      !callback.nil? && !callback.match(VALID_CALLBACK_PATTERN).nil?
+    end
+
+    # Check if the response Content Type is JSON.
+    #
+    # @param [Hash] content_type the response Content Type
+    # @return [TrueClass|FalseClass]
+    def json_response?(content_type)
+      !content_type.nil? && !content_type.match(/^application\/json/i).nil?
     end
 
   end