Space characters are encoded as '+' (#3124)

* Space characters are encoded as '+'

Existing description implies that the specified character classes are the only exceptions to hexadecimal encoding.
I'm trying to match Javascript 'encodeURIComponent' behavior, and expected WebUtility.UrlEncode to provide this. 
However, as spaces are a further exception, this method produces results that are functionally identical to that produced by HttpUtility.UrlEncode.

Might also be useful to mention Uri.EscapeDataString and AntiXss.UrlEncode as well here, as these are now preferred options when looking to encode URIs in a transparent, reversible manner.

* Update xml/System.Net/WebUtility.xml

As requested, removed reference to HttpUtility.
Thanks @davidsh and @mairaw

Co-Authored-By: Maira Wenzel <[email protected]>

Author: Rich

xml/System.Net/WebUtility.xml

@@ -424,6 +424,8 @@
   
 -   `(` and `)` (opening and closing parentheses)  
   
+ Each ' ' (space) character is converted to a `+` (plus) character.
+ 
  For example, when embedded in a block of text to be transmitted in a URL, the characters `<` and `>` are encoded as `%3c` and `%3e`.  
   
  The <xref:System.Net.WebUtility.UrlDecode%2A> method reverses the encoding.