Improve POST bodies support

[danopia]

• Add support for spaces in www-form-urlencoded bodies, by replacing + with spaces before decoding
  • My understanding is that a real + gets encoded as %2B and won't be impacted
• Add support for zero-length bodies, by skipping body reading, and passing a zero-length buffer through instead
  • Only when Content-Length is sent to begin with. Otherwise current behavior is kept.

Both changes tested by submitting <form method="post"> bodies from Chrome.

[kzahel]

Thank you for the fixes.

[danopia]

Thank you for this software - my project would be so much harder if I had to write a whole http stack too! 👍