private static String encodeFormFields(final String content, final String charset) {
        if (content == null) {
            return null;
        }
        return urlEncode(content, charset != null ? Charset.forName(charset) : StringUtils.UTF8, URLENCODER, true);
    }
Encode/escape www-url-form-encoded content.
<p>
Uses the {@link #URLENCODER} set of characters, rather than
the {@link #UNRSERVED} set; this is for compatibilty with previous
releases, URLEncoder.encode() and most browsers.

@param content the content to encode, will convert space to '+'
@param charset the charset to use
@return encoded string
aws-java-sdk-core/src/main/java/com/amazonaws/util/URLEncodedUtils.java