make IsNil guard against nil underlying errors

checker.go

@@ -252,6 +252,12 @@ func (c *panicMatchesChecker) Check(got interface{}, args []interface{}, note fu
 //
 //     c.Assert(got, qt.IsNil)
 //
+// As a special case, if the value is nil but implements the
+// error interface, it is still considered to be non-nil.
+// This means that IsNil will fail on an error value that happens
+// to have an underlying nil value, because that's
+// invariably a mistake.
+// See https://golang.org/doc/faq#nil_error.
 var IsNil Checker = &isNilChecker{
 	argNames: []string{"got"},
 }
@@ -267,6 +273,10 @@ func (c *isNilChecker) Check(got interface{}, args []interface{}, note func(key
 	}
 	value := reflect.ValueOf(got)
 	if canBeNil(value.Kind()) && value.IsNil() {
+		if _, ok := got.(error); ok {
+			// It's an error with an underlying nil value.
+			return fmt.Errorf("error containing nil value of type %T. See https://golang.org/doc/faq#nil_error", got)
+		}
 		return nil
 	}
 	return fmt.Errorf("%#v is not nil", got)

checker_test.go

@@ -1327,12 +1327,12 @@ got:
   []int(nil)
 `,
 }, {
-	about:   "IsNil: error does not guard against nil",
+	about:   "IsNil: nil error-implementing type",
 	checker: qt.IsNil,
 	got:     (*errTest)(nil),
-	expectedNegateFailure: `
+	expectedCheckFailure: `
 error:
-  unexpected success
+  error containing nil value of type *quicktest_test.errTest. See https://golang.org/doc/faq#nil_error
 got:
   e<nil>
 `,

doc.go

@@ -141,6 +141,14 @@ For example:
 	c.Assert("hello world", qt.Contains, "world")
 	c.Assert([]int{3,5,7,99}, qt.Contains, 7)
 
+ContentEquals
+
+ContentEquals is is like DeepEquals but any slices in the compared values will be sorted before being compared.
+
+For example:
+
+	c.Assert([]string{"c", "a", "b"}, qt.ContentEquals, []string{"a", "b", "c"})
+
 DeepEquals
 
 DeepEquals checks that two arbitrary values are deeply equal.
@@ -192,6 +200,16 @@ For instance:
 
     c.Assert(got, qt.IsNil)
 
+As a special case, if the value is nil but implements the
+error interface, it is still considered to be non-nil.
+This means that IsNil will fail on an error value that happens
+to have an underlying nil value, because that's
+invariably a mistake. See https://golang.org/doc/faq#nil_error.
+
+So it's just fine to check an error like this:
+
+    c.Assert(err, qt.IsNil)
+
 JSONEquals
 
 JSONEquals checks whether a byte slice or string is JSON-equivalent to a Go