Do not override the original io.ReadCloser with the gzip reader

The parent should handle the 'closing' of the original / given
`io.ReadCloser`. But, as we are 'creating' a new reader for
gzip-encoded content, the function doing that should be
responsible for closing the reader after all bytes are read.
To be explicit, we should set the new reader to a different
variable, and close it accordingly. Also, prior to this commit,
we did not do any checking to see if there were errors on closing.
This commit does that.

pkg/camo/proxy.go

@@ -104,21 +104,27 @@ func hexEncodeCSSURLs(baseURL *url.URL, hmacKey []byte, css []byte) ([]byte, err
 
 func writeCSSWithResolvedURLs(baseURL *url.URL, contentEncoding string, hmacKey []byte, w io.Writer, r io.ReadCloser) (int64, error) {
 	var err error
+	var gr io.ReadCloser
+
 	if contentEncoding == "gzip" {
-		r, err = gzip.NewReader(r)
-		if r != nil {
-			defer r.Close()
-		}
+		gr, err = gzip.NewReader(r)
 		if err != nil {
 			return 0, err
 		}
+		r = gr
 	}
 
 	css, err := ioutil.ReadAll(r)
 	if err != nil {
 		return 0, err
 	}
 
+	if gr != nil {
+		if err := gr.Close(); err != nil {
+			return 0, err
+		}
+	}
+
 	resolvedCSS, err := hexEncodeCSSURLs(baseURL, hmacKey, css)
 	if err != nil {
 		return 0, err