Add in-tree mspec and ruby/spec

* For easier modifications of ruby/spec by MRI developers.
* .gitignore: track changes under spec.
* spec/mspec, spec/rubyspec: add in-tree mspec and ruby/spec.
  These files can therefore be updated like any other file in MRI.
  Instructions are provided in spec/README.
  [Feature #13156] [ruby-core:79246]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58595 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/rubyspec/shared/complex/numeric/imag.rb

@@ -0,0 +1,26 @@
+require File.expand_path('../../../../spec_helper', __FILE__)
+
+describe :numeric_imag, shared: true do
+  before :each do
+    @numbers = [
+      20,             # Integer
+      398.72,         # Float
+      Rational(3, 4), # Rational
+      bignum_value, # Bignum
+      infinity_value,
+      nan_value
+    ].map{|n| [n,-n]}.flatten
+  end
+
+  it "returns 0" do
+    @numbers.each do |number|
+      number.send(@method).should == 0
+    end
+  end
+
+  it "raises an ArgumentError if given any arguments" do
+   @numbers.each do |number|
+     lambda { number.send(@method, number) }.should raise_error(ArgumentError)
+   end
+  end
+end