fix: Set request content-type encoding to UTF-8

[jclab-joseph]

Do not use new StringEntity without specifying a ContentType.

All non-English characters, are broken because of the code below.

TEXT_PLAIN = create("text/plain", Consts.ISO_8859_1);
DEFAULT_TEXT = TEXT_PLAIN;
public StringEntity(String string) throws UnsupportedEncodingException {
    this(string, ContentType.DEFAULT_TEXT);
}

ContentType.APPLICATION_JSON uses UTF-8.

[jaycee-li]

Thanks for the fix!