Move case to its own example and use nan_value helper

Author: stevegeek

core/float/shared/to_i.rb

@@ -1,11 +1,14 @@
 describe :float_to_i, shared: true do
   it "returns self truncated to an Integer" do
-    -> { (0.0 / 0.0).send(@method) }.should raise_error(FloatDomainError)
     899.2.send(@method).should eql(899)
     -1.122256e-45.send(@method).should eql(0)
     5_213_451.9201.send(@method).should eql(5213451)
     1.233450999123389e+12.send(@method).should eql(1233450999123)
     -9223372036854775808.1.send(@method).should eql(-9223372036854775808)
     9223372036854775808.1.send(@method).should eql(9223372036854775808)
   end
+
+  it "raises a FloatDomainError for NaN" do
+    -> { nan_value.send(@method) }.should raise_error(FloatDomainError)
+  end
 end