`form_urlencoded::ByteSerialize` does NOT conform to the URL standard

[kaffarell]

As mentioned in the URL Standard here the percent-encode of a bytestream is defined as follows:

To percent-encode after encoding, given an encoding encoding, scalar value string input, a percentEncodeSet, and an optional boolean spaceAsPlus (default false):

On step 5.3.1 the spaceAsPlus parameter is used:

If spaceAsPlus is true and byte is 0x20 (SP), then append U+002B (+) to output and continue.

The issue here is that:

1. The url crate does NOT have a spaceAsPlus parameter.
2. The url crate encodes the SPACE character as a '+' per default, although the default behavior should actually be '%20'.

Another issue: #888

[valenting]

Thanks for the bug request. I'm happy to review a PR for this.