[Config] Append the extension to included xcconfigs

Fixes CocoaPods/CocoaPods#1119
Fixes CocoaPods/CocoaPods#1152

lib/xcodeproj/config.rb

@@ -67,8 +67,9 @@ def ==(other)
     # @return [String] The serialized internal data.
     #
     def to_s(prefix = nil)
-      [includes.map { |i| "#include \"#{i}\""} +
-       to_hash(prefix).sort_by(&:first).map { |k, v| "#{k} = #{v}" }].join("\n")
+      include_lines = includes.map { |path| "#include \"#{normalized_xcconfig_path(path)}\""}
+      settings = to_hash(prefix).sort_by(&:first).map { |k, v| "#{k} = #{v}" }
+      [include_lines + settings].join("\n")
     end
 
     # Writes the serialized representation of the internal data to the given
@@ -266,6 +267,22 @@ def extract_key_value(line)
       end
     end
 
+    # Normalizes the given path to an xcconfing file to be used in includes,
+    # appending the extension if necessary.
+    #
+    # @param  [String] path
+    #         The path of the file which will be included in the xcconfig.
+    #
+    # @return [String] The normalized path.
+    #
+    def normalized_xcconfig_path(path)
+      if File.extname(path) == '.xcconfig'
+        path
+      else
+        "#{path}.xcconfig"
+      end
+    end
+
     #-------------------------------------------------------------------------#
 
   end

spec/spec_helper.rb

@@ -20,8 +20,9 @@
 
 require 'rubygems'
 require 'bacon'
-require 'pretty_bacon'
+require 'bacon'
 require 'mocha-on-bacon'
+require 'pretty_bacon'
 require 'pathname'
 
 ROOT = Pathname.new(File.expand_path('../../', __FILE__))