net/http: correct error message for incorrect Body length

Fixes #8140.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/131900044

src/pkg/net/http/requestwrite_test.go

@@ -280,7 +280,7 @@ var reqWriteTests = []reqWriteTest{
 			ContentLength: 10, // but we're going to send only 5 bytes
 		},
 		Body:      []byte("12345"),
-		WantError: errors.New("http: Request.ContentLength=10 with Body length 5"),
+		WantError: errors.New("http: ContentLength=10 with Body length 5"),
 	},
 
 	// Request with a ContentLength of 4 but an 8 byte body.
@@ -294,7 +294,7 @@ var reqWriteTests = []reqWriteTest{
 			ContentLength: 4, // but we're going to try to send 8 bytes
 		},
 		Body:      []byte("12345678"),
-		WantError: errors.New("http: Request.ContentLength=4 with Body length 8"),
+		WantError: errors.New("http: ContentLength=4 with Body length 8"),
 	},
 
 	// Request with a 5 ContentLength and nil body.

src/pkg/net/http/transfer.go

@@ -228,7 +228,7 @@ func (t *transferWriter) WriteBody(w io.Writer) error {
 	}
 
 	if !t.ResponseToHEAD && t.ContentLength != -1 && t.ContentLength != ncopy {
-		return fmt.Errorf("http: Request.ContentLength=%d with Body length %d",
+		return fmt.Errorf("http: ContentLength=%d with Body length %d",
 			t.ContentLength, ncopy)
 	}