Constrain actual in be_within matcher to values that respond to -

instead of requiring a specific type.

- `Time`, for example, is a legit alternative.
- Fixes issue introduced by #145

lib/rspec/matchers/built_in/be_within.rb

@@ -8,8 +8,8 @@ def initialize(delta)
 
         def matches?(actual)
           @actual = actual
-          raise needs_expected unless defined? @expected 
-          raise needs_numeric  unless @actual.is_a? Numeric 
+          raise needs_expected     unless defined? @expected 
+          raise needs_subtractable unless @actual.respond_to? :-
           (@actual - @expected).abs <= @delta
         end
 
@@ -32,8 +32,8 @@ def description
 
         private
 
-        def needs_numeric
-          ArgumentError.new "The actual value (#{@actual.inspect}) must be of a `Numeric` type"
+        def needs_subtractable
+          ArgumentError.new "The actual value (#{@actual.inspect}) must respond to `-`"
         end
 
         def needs_expected

spec/rspec/matchers/be_within_spec.rb

@@ -49,6 +49,10 @@ module Matchers
           matcher.failure_message_for_should_not.should == "expected 5.49 not to be within 0.5 of 5.0"
       end
 
+      it "works with Time" do
+        Time.now.should be_within(0.001).of(Time.now)
+      end
+
       it "provides a description" do
         matcher = be_within(0.5).of(5.0)
         matcher.matches?(5.1)
@@ -62,9 +66,9 @@ module Matchers
         )
       end
 
-      it "raises an error if the actual value is not of a Numeric type" do
+      it "raises an error if the actual does not respond to :-" do
         expect { be_within(0.1).of(0).matches?(nil) }.to raise_error(
-          ArgumentError, /The actual value \(nil\) must be of a `Numeric` type/
+          ArgumentError, /The actual value \(nil\) must respond to `-`/
         )
       end
     end